THE APPLICATION OF LIFE CYCLE COSTING TO NEW AND
EXISTING PIPELINE SYSTEMS
Municipalities are under continuous pressure to reduce the capital costs of new service delivery schemes whilst maintaining, with reduced capital and human resources, the efficiency and integrity of existing infrastructure.
Life Cycle Cost Analysis can be utilised as an effective design and maintenance management tool to reduce maintenance requirements and maximise energy and delivery efficiency of water and wastewater pipeline systems.
Pumps and pipeline components have historically been purchased based on lowest capital costs and occasionally, the reliability and costs of spares. In addition, components such as valves are purchased with one particular operation in mind and without any due consideration of how these devices will perform over the complete operating cycle of the system. The initial purchase price of pumps and components is generally a small part of a system’s Life Cycle Cost. However, they need to be carefully matched to each other, to ensure the lowest energy and maintenance costs and the general extension of equipment life.
This document highlights the impact that pipeline component selection has on Life Cycle Costing and introduces the least intrusive and most beneficial manner of implementing Life Cycle Cost Management and maintenance to existing pumps and pipelines.
DEFINING LIFE CYCLE COSTING AS APPLIED TO NEW AND EXISTING PIPELINES
Conventional approach to analysis of an economical pipeline design decision normally involves comparison of the total initial cost of the installed system but is seldom followed by a cost comparison of the commissioning, energy consumption, operating and maintenance cost of the equipment, and factors such as human resource requirements which may impact on the future functionality, durability and performance of the system.
Life Cycle Costing as applied to water and wastewater pipelines and, as defined in this document, is beyond its traditional narrow definitions of pipeline diameter selection or fashionable pump analysis. It is defined (or redefined) into an all embracing discipline that considers the sum of all costs during the total lifetime of a pipeline system. It is an approach that balances the functionality of a system with the preservation of limited valuable resources, such as power, water and capital, from initial design to final disposal whilst finding the most beneficial use of the system over this time period. In addition, it is a discipline that is applicable to both new and existing pipelines and can be applied to the latter in phases to suit budgets or, holistically in the least intrusive manner and funded by overall savings generated from its application.
GENERAL PIPELINE LIFE CYCLE COSTING DESIGN CONSIDERATIONS
Life Cycle Costing drives all design decisions to money related decisions - some factors are more tangible than others such as initial capital and power consumption while others such as maintenance requirements are more condition, quality of equipment or human resource related. All these factors have to be considered individually for every component selected then matched to the relevant sub systems which in turn need to be evaluated collectively to determine the total Life Cycle Costing of a system. Typical factors to consider for a pipeline system are:
Initial Capital Costs
Initial pipeline capital costs are largely driven by the community that the system will service. Most new pipeline development is targeted for rural areas. Capital cost reductions and future reduced maintenance requirements for these schemes can be realised by consideration of the realistic initial and future consumption requirements based on realistic population growth estimates; the design of treatment works, pumpstations and reservoirs in modular units; the use of appropriate construction specifications for underground pipelines and; the reduction in the length of reticulation pipelines where possible.
Life Cycle Cost Analysis requires that the lowest initial capital cost should be weighed against factors such as surge and waterhammer, long term pipeline efficiency, pipeline maintenance requirements and pump energy consumption so practicality is never compromised for perceived savings - every aspect must be grounded in practicality.
Power consumption is generally the most highlighted aspect of Life Cycle Costing. This is due to the fact that estimated power consumed by pumps accounts for nearly 14% of the world’s energy demand. More than 70% of the cost of operating a pumping main typically goes into the cost of power. Most municipal services, including the conveyance of water and the processing of wastewater, consume large amounts of power.
ESKOM has, due to South Africa’s rapid rising demand for electricity, embarked on a Demand Side Management programme (DSM) to curb power consumption by providing a financial benefit to the efficient and intelligent use of this resource which is based on the principle that a kW saved equates to a kW generated. DSM, in the context of pumping mains, can either be load shifting or actual power savings based on the reduction of energy consumed by pumps and pipeline equipment. ESKOM’s DSM programme will fund up to 50% of a Life Cycle Costing programme focussed on power savings.
Surge and Waterhammer
Surge and waterhammer in a pipeline system is inevitable if design precautions are not taken. These destructive phenomena can be aggravated by the nature of the profile, the pipeline flow velocity, pipe laying practice and, pipeline components selection such as pumps and valves.
Pipeline components such as valves are purchased with one particular operation in mind and without any due consideration of how these devices will perform over the complete operating cycle of the system. Careful matching of pipeline components to the pipeline and the limitation of flow velocities by design and operating practices can substantially reduce the occurrence of surge and waterhammer whilst still ensuring the lowest energy and maintenance costs and the general extension of equipment life.
Surge and waterhammer is often associated with bulk water pumping mains but frequently occurs in gravity and networks systems or wherever a control device is utilised. These factors can be resolved by attention to design.
The cost of maintenance can be elusive partly due to the differences in municipalities’ maintenance policies, the availability of maintenance staff, the difference in system design, and the difference in quality, construction and maintenance requirements for equipment of the same function. However, to ignore the inclusion of maintenance costs because they are difficult to evaluate is to skew cost comparisons and decisions to initial capital costs only.
The largest control over maintenance requirements is in the initial design and installation stage by adherence to basic principles such as pump suction design, pipeline profile design, specification of suitable materials and access to equipment that requires maintenance.
Technology and Human Resources
A typical municipality pipeline distribution system can extend tens of kilometres and comprise hundreds of valves, pumps and water meters. The challenge is, because of reduced resources, to monitor these devices remotely through a cost effective and efficient remote monitoring and control system. Data gathered in this manner needs to be accessed in a user friendly format and translated into information that will drive down life cycle costs by assisting in the determination of scheduled maintenance programmes, providing alarms for failed components and allowing for the remote adjustment of flow and pressure on critical components.
GPRS is the acronym for General Packet Radio Service, a technology that is particularly suited for sending and receiving data in a variety of formats. This technology has been incorporated into a series of pipeline components and allows for low cost remote monitoring of equipment such as valves, pumps, watermeters. Communication can be to a central computer or over the internet, and can be used for incoming alarms and status reports as well as for archiving and exporting of data. The units that house the technology are self-contained, require no external power source, have a minimum five year battery life and an alarm that reports to the management system when the battery is deemed to be nearing the end of its useful life.
Technology can and should be utilised with great effect in overall Life Cycle Cost management programmes. It is important to note that whilst available technologies assist in scheduling maintenance and in reducing operating cost, they need to be implemented and monitored and actions taken by relevant on information received. These functions can also be cost effectively outsourced with payments for the outsourcing extracted from overall Life Cycle Cost savings.
APPLYING LIFE CYCLE COSTING THROUGH DESIGN CONSIDERATIONS AND COMPONENT SELECTION
Every water or wastewater conveyance system is unique and should be analysed accordingly. Each system consists of separate but mutually dependant sub systems namely: a hydraulic sub system, a control sub system and in the case of pumping mains, an electrical sub system. Pipeline can also be analysed by splitting the pipeline from initial intake to final discharge into subsystems - these would typically be: suction design, pumps, discharge pipework and valves and fittings.
Analysis of these sub systems must include, in addition to obvious capital costs, the installation costs, maintenance requirements, component material selection and design of components. These are influenced by engineering design and operating requirements. Operation defines wear, component selection defines strength and performance, and frequency of usage determines life of the components - all these factors relate to money decisions.
System deterioration is inevitable and deterioration drives costs. Deterioration can be slowed by preventive maintenance to prolong equipment life and operating costs can be reduced substantially through structured refurbishment and component replacement actions. Analysis therefore for existing systems, should be based on long term cost reduction vs. short term expenditure i.e., at which point does the reduced operating cost cover the initial refurbishment cost and begin to pay dividends.
Presented is a practical overview of how Life Cycle Costing can be applied to new and existing pipelines through the evaluation and selection of some pipeline components along the lines highlighted above.
PUMP SUCTION DESIGN
Incorrect pump suction design and/or installation often contribute to high maintenance and energy consumption costs. Most pump cavitation problems are directly related to poor suction design. Friction losses in a suction system must be controlled within acceptable limits. Air intake and accumulation, and vortex action of fittings and valves should be avoided at all costs. Design and installation factors in line with Life Cycle Costing factors to consider are:
Sizing of Suction Piping - Flow velocity in the suction system should be less than 2.4 m/sec. Suction pipe sizes should be at least one or two sizes larger than the suction nozzle on the pump. The entire suction piping system should incline slightly upward toward the pump. A suggested minimum slope is 20mm per metre.
Sizing of Suction Piping - Flow velocity in the suction system should be less than 2.4 m/sec. Suction pipe sizes should be at least one or two sizes larger than the suction nozzle on the pump. The entire suction piping system should incline slightly upward toward the pump. A suggested minimum slope is 20mm per metre.
Eccentric Reducers - Eccentric reducers must be used as a transition between the suction line and the pump inlet nozzle and should be attached directly to the pump nozzle whenever possible. In addition, the reducer should be installed with the horizontal section to the top so as to prevent the accumulation of air.
Suction Inlet - The suction pipe inlet should be submerged at least four pipe diameters and be at least one pipe diameter off the bottom. In situations where there is not enough submergence, an anti-vortex plate should be used to prevent air from being drawn into the suction system. Up to 10% of air by volume can be drawn into a system due to poor inlet design.
Suction Bends - A straight piece of pipe at least seven pipe diameters long should be used between the pump suction nozzle and any bend. An elbow attached directly to the pump suction causes unequal thrust due to the liquid filling one side of the suction chamber and impeller eye more than the other.
Suction Isolator - The use of butterfly valves as suction isolators should be avoided because of their potential to create turbulence. Butterfly valves are often selected on suction applications as assumed cost effective alternatives to gate valves. However, the additional pipework needed between the pump suction nozzle and a butterfly valve to prevent any turbulence obviates perceived financial benefits.
Gate valves, because of their extremely low headloss characteristics, are preferred suction line isolators. In addition, Resilient Seated Gate valves in diameters up to DN600 are often more cost effective than double flanged butterfly valves. Further, rising spindle gate valves make it easier for maintenance staff to see if the isolator is open before restarting a repaired pump and so preventing motor burnout or, in severe instances, pump explosions.
Air Discharge - It is imperative to prevent the introduction of air into the suction side of the pump system as air can result in surges, efficiency problems and pump cavitation. In addition to precautions on suction inlet design and the installation of concentric nozzles, other considerations should be the installation of a small air release valve on large pump volutes; the installation of an air release valve on long suction lines; or, systems with negative suction heads.
Negative Suction Heads - Pump capacity and efficiency is reduced by excessively high suction lifts. Inlets should be fitted with foot valves to automatically keep the suction line primed. The selection of a foot valve should ensure low headloss as the variance between the headloss characteristics of varying foot valves could be as high 650%.
A strainer should always be installed on the suction system. The selected strainer must have an open area at least three times the area of the suction pipe to minimise headloss.
For a pump to operate accurately, it needs to be matched to the system duty. A system operating off a pump’s Best Efficiency Point (BEP) results in a number of pump maintenance and efficiency problems including low bearing and seal life, cavitation, high energy consumption and recirculation. Pump Life Cycle costing can be reduced by:
Installation of Real Time Monitoring Equipment - Pump suppliers, consulting engineers and end users in the form of municipalities and water authorities need to request as standard, the installation of pump monitoring equipment to accurately track a pump’s performance from time of installation; to identify issues such as oversizing duty and volumetric loss; and to accurately predict maintenance scheduling programmes.
Correct Sizing of Pumps - A pump once installed seldom matches the design curve. This is largely due to the selection of pipeline components, changes in the pipeline profile and/or safety factors added to the pump between the point of design and actual installation. It is predicted that more than 90% of pump systems are inaccurately or inefficiently designed, with consequent increases in cost of ownership. Many of these factors can be resolved in the initial design stage by adherence to principles outlined in this document and through reference to other sources.
Variable Speed Drives - Variable speed drives provide smooth operation over a wide speed range. The benefit of variable speed drives are:
Significant savings can be made in power and maintenance requirements through the appropriate application of variable speed drives. However, caution should be applied to their selection as they should be totally avoided where constant speed drives will suffice; where variable demands do not occur; and in low friction systems where a small change in speed may result in a very large change in discharge.
Motor Selection - The performance of motors has an impact on the overall power consumed. More electricity efficient motors should be utilised and worn motors should be replaced with electricity efficient motors rather than rewound.
Power Shifting -Savings in power can be achieved through the shifting of pumping times to off peak demand periods. Utilising this method may require the expansion of water storage facilities. Overall saving should be sufficient to cover changes in storage facilities. It should, also be considered that in mid summer, peak pumping may be required.
Maintenance - All equipment deteriorates with time and needs to be maintained. The deterioration in performance of pumps is directly related to reduced efficiency and increased power consumption. Poor performing pumps can through scheduled Life Cycle Costs driven refurbishment, dramatically reduce power consumption costs. Refurbishment factors, dependant on the system and pump conditions should consider the following:
DISCHARGE PIPELINES, GRAVITY MAINS AND DISTRIBUTION NETWORKS
Pipelines contribute the largest initial capital cost and, if material of construction is not suitable for the application or, the pipeline is not laid to applicable standards, will contribute to the largest overall maintenance costs. Pipeline material selection is an optimisation process, and the material selected for an application must be chosen for the sum of its properties. Considerations beyond initial capital costs and availability should include:
Corrosion Resistance - The selection process for a corrosion resistant pipeline material should consider compatibility with environmental conditions. Additional factors should include fabrication and installation costs, support system complexity and cathodic protection requirements - applicable to ferrous pipeline materials.
Sizing - The sizing for any piping system consists of two basic components namely; fluid flow design and pressure integrity design. Fluid flow design determines the minimum acceptable diameter of the piping necessary to transfer the fluid efficiently. Sizing of pipelines should take into account the pipeline’s internal diameter and headloss characteristics and, the pipeline profile in line with pump selection.
Pressure integrity design determines the minimum pipe wall thickness necessary to safely handle the expected internal and external pressures and loads. Care must be taken, when stepping up pressure ratings for pipeline materials with constant outside diameters, that power consumption costs and velocities are not increased.
Power Consumption - Power consumption forms 75% of a pumping main’s operating costs. The amount of power consumed by a pipeline’s pump/s is dependent on the pipeline profile, pipeline material, pipeline diameter and the pump/s selected. Headloss across pipeline components or any other restriction in a pipeline will also have a major effect on the amount of power consumed.
Pipeline materials with the same headloss characteristics but different internal diameters for the same nominal size may therefore have an effect on the amount of power consumed. Similarly, a pipeline material with a higher headloss characteristic but a larger internal diameter for the same nominal size may in fact, have a lower power consumption than a pipeline material with a lower headloss characteristic but smaller internal diameter.
Coatings such as bitumen on fittings can increase power consumption dramatically even if the main pipeline material has low headloss characteristics.
Air Release and Vacuum Protection - It is important to release air in a controlled manner spacing air valves in accordance to the pipeline profile and diameter. However, it is important to provide adequate accumulators in the form of unequal tees with the vertical branch of the tee 1/3rd the diameter of the main pipeline as air may otherwise be swept past the air valve and accumulate downstream creating surges and restrictions in flow and high power consumption as pumps are forced to work at higher heads in order to overcome the restrictions.
Pipelines need to be filled at moderate rates of no more than 0.5 m/sec to prevent surges from rapid air release. Surges created by rapid air release are cumulative and concentrate at points of weakness such as reductions in pipe class, fittings which may be of a lower standard than the surrounding pipes, near line valves or tapers and in branches with closed ends. These surges, in addition, result in the structural failure of pipes due to the combined effect of the surge pressures which crack protective pipe linings and the retained air pockets which promotes corrosion.
The selection of pipeline components such as air valves, check valves and end line level control valves have a major effect on surge and waterhammer, as preventative mechanisms in some designs and as the primary source of these phenomena in others.
Headloss across pipeline components or any other restrictions in a pipeline will have a major effect on the amount of power consumed. Some components provide a higher resistance than others of a similar function. In addition, the maintenance requirements of some designs are higher than those of others. All these factors when considered in the initial design stage and under the umbrella of Life Cycle Costing relate back to money decisions.
Air is present in any “empty” pipeline and needs to be discharged in order to effectively hydraulically pressurise the system. In addition, water constitutes 3% of air by volume. This air will come out of solution due to changes in temperature and pressure and needs to be discharged from time to time in order to prevent corrosion, flow restrictions and surges. Further, air needs to be introduced into a draining system to prevent pipe collapse in the case of large diameter thin walled pipe or, the drawing in of seals, which will result in leaks when the pipeline is re-pressurised.
Air valves are utilised to automatically perform these functions. However, incorrect air valve selection and application can be the root cause of many pipeline failures. Factors to consider are:
Vacuum Protection - Air valves need to be sized sufficiently for air intake in order to minimise the occurrence of vacuum in a draining pipeline. Vacuum, dependant on pipeline material and diameter should be limited to two to three metres negative differential.
Municipalities in South Africa experience on average, waterlosses of 27%. Insufficient vacuum protection contribute to these losses as too low a negative pressure will result in the pipeline seals being drawn in and debris entering behind the seals and subsequent re-pressurising of the system will result in leaks.
Air Discharge - An air valve discharging air at high differential pressures and velocities will, on closure induce high and damaging transient pressures. This is due to the water flow entering the valve suddenly being arrested by the large orifice control float sealing. The effect on the pipeline dynamics is equivalent to the rapid closure of an isolating valve.
The magnitude of transient pressure rise created on closure is dependent on the valve size, length of pipe, differential pressure across the large orifice on closure and the bulk modulus of the water. Effects of transients created in this manner will progressively undermine the integrity of the system and can result in premature pipeline failure.
Air valves should therefore be fitted with automatic surge dampening devices or sized to limit the discharge differential across the large orifice from 5 to 10 kPa differential pressure, in order to prevent damaging high pressure transients from occurring. The cost of selecting an air valve with a surge dampening device, or sizing it sufficiently to limit the differential pressure across the large orifice, is minimal as air valves constitute approximately 1% of the total value of a pipeline project but can contribute significantly to pipeline damage if not correctly sized.
Small orifice air valves should be utilised in front of watermeters at changes in pipeline diameters and along the length of long pipelines to release pressurised air that may come out of solution and accumulate in the system. It has to be ensured that these valves can release air at the rated pressure of the pipeline as failure to do so, will result in pipeline restrictions, surges and other air related destructive phenomena.
Over Specification of Air Valves - Air valves are often over specified based on the wrong interpretation of the pipeline profile. This results in valves positioned too closely together. The impact of this practice, beyond the initial capital cost of the valve chambers, isolators and fabricated fittings to install the additional air valves, is the creation of air valve slam and its subsequent damage to the system. Air valve slam occurs because the first air valve in a series of closely positioned air valves is often large enough to discharge a slug of air effectively resulting in insufficient air volume to switch the floating orifices in the subsequent air valves to the anti surge or non slam mode. Water entering these air valves slam the control float into the large orifice, generating high pressure transients. Damage of this kind is often reflected in air valve leakages or leakages at pipe and fitting joints.
Check Valves (Non Return Valves)
Check valves, also known as Non Return valves are commonly installed on the discharge side of the pump, to prevent drainage of the system and backflow through the pumps upon pump trip.
This component, like most critical pipeline components should be selected taking the broader life cycle costing of the system into account and should be evaluated with due consideration of the following:
Reverse Flow Effects and Surge and Waterhammer - Ideally a check valve should open with the onset of upstream pressure and allow flow through the valve with minimal resistance. The valve should close at the instant of zero flow velocity and remain positively closed during minor pressure surges, and should resist back pressure without leakage.
However, many of these designs only close on flow reversal. Reverse flow and subsequent reverse rotation of pumps can cause damage to pump motors. In addition, reverse flow causes severe surge pressures in the suction side in the case of positive suction head applications. Further, reverse flow causes high transient pressures as the column of water is already in motion and stops abruptly as the check valve closes.
System Design - Each system design is unique, as the factors that may cause Check Valve Slam in one system may be totally different to those that cause slam for the next. Factors to consider are:
Maintenance Requirements - System down time and pipeline repairs due to check valve damage should be
considered. Wear in check valves are greater in designs that are hinged as the action of the disc continuously “riding”
on the flow rapidly wears the valve shaft and locating lug.
Maintenance Requirements - System down time and pipeline repairs due to check valve damage should be considered. Wear in check valves are greater in designs that are hinged as the action of the disc continuously “riding” on the flow rapidly wears the valve shaft and locating lug.
Check valves that incorporate springs in their designs, but do not have the benefit of a positive opening characteristic, are subject to spring cyclic failure as the disc flutters in the vortex caused by the fluid moving past the discs. Double Door, Poppet and Co-Axial check valves are especially susceptible to this form of failure.
All valves that do incorporate springs in their design should be selected very carefully to ensure that the goal of non slam is not compromised by too high a headloss or, that the spring selected is not too weak to ensure positive and rapid closure thereby creating high transients in the system as the closing member would be slammed closed by flow reversal in such instances.
Headloss and Power Consumption - Check valves have a substantial effect on power consumption. There is a major difference in headloss characteristic between manufacturers of similar designs and an even bigger difference between different design types with comparative variances in headloss of 300% or more. Check valves with low headloss characteristics will often cost more but the difference in cost is often realised within a few months to a year or two by the savings in power consumption. From that point on, it will pay dividends.
Control Valves Control valves form an essential part of any pipeline network system as the valves are utilised to automatically control flow, pressure and water levels and, to alleviate surges. Some designs have inherent higher maintenance requirements than others and the misapplication of these devices can result in pipeline failure. Life Cycle Cost factors to consider are:
Speed Controls - Pressure reducing valves should be fitted with speed controls as the rapid reaction of the valves, especially those utilised in pressure management applications, create transients that cause system damage.
Surge and Waterhammer - Pressure relief valves are used effectively to counter over pressures, but are incapable of handling negative pressures. The operation of this device is such that the positive wave reflects off the reservoir and returns to the device where it is dumped to atmosphere. However, a damaging negative pressure develops along the length of the pipeline which needs to be countered by the use of good air valves and should not therefore be considered as a standalone surge protection strategy.
Maintenance - The scope of operation of a control valve can be varied with a single valve capable of performing multiple functions. However, the greater the complexity of the valve control units, the greater the maintenance requirement, and the higher the likelihood of valve failure. Complex functions should therefore be distributed as far as possible over more than one valve with no more than two functions per valve.
Pressure Management and Leakage Reduction
Water network systems are designed to deliver a minimum pressure to all points in a 24 hour period. Demands fluctuate throughout the day and are lowest during late evenings and early mornings. The low demand can result in bursts due to the high system pressures and low friction losses present at these times.
Most existing systems leak due to poor vacuum protection, corrosion effects, resettlement of the soils that the pipes have been laid in and damages as a result of surge pressures. Leakage rates are therefore higher during times of high pressure and low demand.
Pressure management has developed as a methodology to reduce nighttime pressures and so, reduce water leakages and is often applied as an independent strategy to pipelines. However; pressure management should be seen as part of an overall Life Cycle Costing strategy for existing pipelines. Considerations in applying pressure management are:
Life Cycle Costing as a design tool can reduce both the initial capital costs and long term operational and maintenance costs through the principle of bringing every pipeline decision back to monetary and operationally practical decisions. Systems need to be evaluated holistically and every component matched to the pipeline over the complete operational cycle of the system. Valves and fittings are major contributors to pipeline headlosses, maintenance requirements, surge and waterhammer and other destructive pipeline phenomena. These components cannot therefore be treated generically during initial design but should be evaluated along the guidelines presented in this document.
Critical components such as pumps, control valves and watermeters should be fitted as standard with remote monitoring equipment to accurately monitor system performance and schedule maintenance when required.
Existing pipelines run inefficiently due to a number of factors. Major power savings can be realised and maintenance requirements reduced on these systems through a systematic Life Cycle Costing refurbishment programme that targets, in the least intrusive manner, the most critical components such as the suction pipework, valves and pumps.
Component monitoring and refurbishment programmes can be outsourced and funded through savings derived from reduced operating and power consumption costs. Refurbishment programmes can be further subsidised by ESKOM’s DSM programme. Municipal staff can increase knowledge on pipeline components by linking every project to a training programme for their staff. Local employment can be stimulated by linking refurbishment programmes to the employment of local staff for semi skilled or unskilled functions.
Life Cycle Costing should be integrated into every new pipeline design decision making process and should be applied to every existing pipeline if systems are to be managed with reduced resources.