Pipes that burst as a result of transients and surges within a piping network are very common problems throughout the world. Not only are the maintenance costs of these repairs extremely expensive, but when you add in possible litigation for third party damages, combined with the potentially significant value (cost) of the lost water, you can quickly realize the significance of what may appear simply as ?another pipe burst.? It is common for water utilities that serve a typical population base of 1 to 1.5 million people to quote pipe burst occurrences between 500 to 1,000 incidents per year. Maintenance repair costs quoted at thousands of U.S. dollars per incident are common.
Surges, or transients, are the result of a rapid change in liquid velocity within a pipeline. This stored energy ? released as pressure ? can destroy fittings, pipes, valves, instrumentation and pumps. The associated pressure waves travel the length of the pipeline (upstream or downstream), of the offending device (pump, valve, etc.) and then reverse direction. The waves move at a constant speed until they meet a boundary or barrier. The reflected and incident waves superimpose to produce a more complicated wave pattern that includes double-peaks and double-troughs. The consequence of inattention or improper protection from surges or transients could be a ?pipe burst? or equipment failure and result in damage, water loss or litigation.
Transients, surges and the resulting pipe bursts can be caused by numerous events. Loss of power at a pump station, pump station PLC malfunction, single-speed pump motors without adequate pump control valves, or the rapid closure of gate valves or butterfly valves within the distribution system are a few of the more common causes.
This paper describes a variety of situations, while citing examples of transients/surges and the resulting damage. It also presents a variety of pressure relief options for water utilities and educates interested parties on strategies for the overall reduction of pipe bursts and the resulting savings in water. The paper also refers to the associated costs/risks of failures.
Introduction to Transient Behavior
?Water hammer? is a term most of us are familiar with and is the name we often associate with transients. This term is given to the pressure fluctuations that can develop in a pipe when the flow of liquid is suddenly changed. The most common example of water hammer is when pipes in houses rattle when taps are suddenly closed.
The sudden closure of a valve at the end of a long pipeline causes an instant surge at the inlet of the valve. This high pressure expands the pipe and stores energy. This stored energy causes a positive pressure wave that travels upstream. The resulting pressure can be large and can put pipeline integrity at risk. Conversely, a sudden flow stoppage initiated at the upstream end, caused by a pump trip (e.g. power outage) for example, results in a negative pressure wave traveling downstream. The surge then returns as a high pressure wave.
Consequences of Transient Damage and Pipe Bursts
Transient damage and pipe bursts can be extremely costly and result in excessive water loss. A partial list of consequences could be:
- Inconvenience and interruption of service to customers (no water)
- Disruption of traffic due to road closures
- Water loss and associated costs
- Pump and manifold damage (maintenance and repair costs)
- Damaged pipes within the utility (maintenance and repair costs)
- Litigation from third parties
Transient Analyses, Formulas and Modeling
Engineering firms use two key formulas in the review of transients ? the Surge Formula (Joukowsky Equation) and the Critical Time Formula. Without ?in-house? expertise, it is always strongly recommended utilities retain a qualified engineering firm to provide a professional review of the system in question. A detailed analysis is required to track pressures subsequent to the initial wave interactions with the various boundary elements (reservoirs, pumps, valves, etc.) in the system. In some cases, these wave interactions and the behavior of boundary elements can result in unacceptably high pipe pressures.
Changing Factors and Life of System
It is well documented that pressure has a direct influence on water loss, leakage and pipe bursts. As pipes deteriorate through age (and possibly corrosion), and other local and seasonal factors, the ?failure? pressure gradually reduces until burst frequency starts to significantly increase. The first step in pressure management (with the aim being a reduction in lost water) is to check for the presence of surges or variations and, if they exist, reduce the range and frequency of both.
Transient Solution Overview
Numerous devices can be used to mitigate transient pressures. However, without analysis, there cannot be a design. The first step is to determine if there is a potential problem of surges in a pipe system that will exceed the pressure limitation. This can be accomplished by numerous means, including the use of a variety of available transient modeling software. If transient problems exist, specifically designed transient data loggers can detect peaks and troughs by logging data in minute timeframes (one second intervals, if required).
Numerous devices can also be considered for mitigation of surges. These solutions can also be used in combination. They are not listed in any particular order of effectiveness or cost (listed below). Each solution is specific to the system being analyzed in detail by an engineer who is fully qualified in this discipline.
Solutions can be modeled in modern software and will have varying degrees of success in avoiding catastrophic or repetitive damage type scenarios. Some solutions depend on pneumatic, hydraulic, electrical power, or fluids being available to ensure their effectiveness. No solution should be integrated without a detailed analysis of the particular system.
All devices used to mitigate transient surge pressures or vacuum need to be maintained. The devices should be given the same level of attention as safety relief valves. If the surge mitigation device does not perform when required, surge pressure events will occur. The devices and mitigation processes should be fully documented, routinely tested and properly labeled. Some examples of devices and strategies are as follows:
- Stronger pipe work to withstand pressure surges
- Reroute piping
- Additional pipe supports
- Change of pipe material to one with a lower modulus (i.e. thermoplastic pipe materials)
- Flow control valves
- Air/vacuum release valves
- Intermediate check valves
- Non-slam check valves
- Bypass valves
- Gas accumulators
- Liquid accumulators
- Surge tanks
- Surge shafts
- Surge anticipation valves
- Relief valves
- Bursting discs
- Increase diameter of pipeline to reduce average velocity
- Variable speed drives
- Soft starters
- Valve closure and opening times
- Increased inertia of pumps and motors (i.e. flywheels or by selection)
- Minimize resonance hazards with additional supports
- Investment in additional engineering
Non-Diaphragm Operated Control Valve Solutions
Rupture Discs and Burst Discs
Rupture discs and burst discs are made from numerous materials; however, graphite and stainless steel sheet are the most common materials. Rupture discs are designed to fail (break or shatter) at a predetermined pressure and are very effective at managing overpressures or transients. Rupture discs are commonly located off a tee and have some form or isolation valve upstream of the tee. An isolation valve is typically manually closed when failure occurs, allowing replacement of the rupture disc. A rupture disc is retained between two flanges and, when it does fail, it normally discharges the water to atmosphere. It is critical that the discharge at time of failure is anticipated and the system is designed for the eventuality. When the rupture disc fails, water is discharged from the ruptured disc.
Surge Tanks and Surge Drums
Surge tanks, or surge vessels, are typically constructed from steel and are partially filled with air (the balance being water). Normally, the tank is filled with 50 percent air and 50 percent water at operating pressure. The air pressure is supplied by an air compressor and associated controls. Water is allowed to freely flow out of the tank to prevent column separation. When the transient returns up the pipeline, the surge tank acts like a relief valve to prevent overpressure. Sizing of flow into the tank is critical and must be accurately controlled and is based on transient modeling.
Often, a properly sized orifice plate is the best solution for the return flow into the surge tank. A surge tank or vessel is a viable solution for the control of transients or surges. However, a few areas of concern must be considered. The space required for a surge tank can be significant, as surge vessels can be very large, unsightly and expensive. Maintenance of surge tanks is also critical, as surge tanks can become waterlogged if they are not properly serviced. For example, if the air compressor and associated air system are not maintained, the tanks can fill with water, rendering them ineffective to counter transients in the system. Other concerns occur in extreme climates that are prone to freezing. In these cases, surge tanks should be located indoors to prevent freezing, which can be cost prohibitive. Theft of air compressors associated with surge tanks and required security are other factors to consider.
Diaphragm Operated Control Valve Solutions
Pressure Relief Valves
A diaphragm style, pilot operated pressure relief valve responds only to a high pressure wave. If the standard operating pressure is 105 psi (7 bar), you can choose to set the relief pressure on the pilot slightly higher: for example, at 115 psi (7.5 bar). If an overpressure occurs (fast closing valves, etc.), the relief valve quickly opens and discharges water to atmosphere and reduces the overpressure. When the pressure has stabilized, the pressure relief valve closes and normal operation of the system resumes.
The selection and sizing of pressure relief valves are very important. Generally, a pressure relief valve should be sized for one quarter (25 percent) of forward flow. This guideline requires a detailed analysis by a transient specialist who looks at all factors before making a final, informed selection. The pressure relief valve is normally mounted off a tee on the header discharging to atmosphere.
A relief valve with an intermittent flow capacity of 25 percent of the maximum flow in the main pipeline is a good initial selection. If larger valves are needed, surge anticipating or ?rate of rise? valves are often a better choice. Pressure relief valves do not have to be limited to pump stations and can be strategically located anywhere in the distribution system to deal with overpressures and transients.
It is also important to consider the discharge that is released from the relief valves when they react to the overpressure. The discharge can often be chlorinated water that poses dire consequences for fish bearing streams, landscaping, etc. A well-considered approach to the proper management of the discharged relief water is always required.
Surge Anticipating Relief Valves
Standard relief valves only start to open when the system pressure exceeds the pilot setting. Should the surge increase rapidly, standard relief valves may not have time to open and will then be ineffective.
Surge anticipating relief valves react to the period of low pressure after a power failure. A second pilot opens the valve whenever the system pressure falls below its set point. How low the second pilot is set should be carefully considered, but it should always be set lower than static less full flow friction. By sensing the dip in pressure, the surge anticipating relief valve has time to be at least partially open when the wave returns, thereby reducing the overpressure. The stable pressure after operation must exceed the pilot setting to ensure the valve closes and does not drain the entire pipeline. Correct sizing of the surge anticipating relief valve is very important and oversizing the surge anticipating relief valve can be a common mistake. Consulting with the valve manufacturer, or working with a knowledgeable surge consultant, is always recommended.
Surge anticipating relief valves need significant static pressure to operate properly. A minimum of 100 ft (30 m) is typical. Sizing concerns for surge anticipating relief valves are similar to pressure relief valves. Selection and sizing are usually based on one quarter of forward flow. This should be verified by a transient specialist and surge anticipating relief valves must be sized correctly.
Surge anticipating relief valves are often a good selection when design criteria calls for valves 6 in. (150 mm) or larger. This style of valve is often found on larger distribution and trunk mains. One of the most common problems with surge anticipating relief valves is when they are oversized and do not recover or close when the transient is completed. This is caused by friction from high flow returning due to the static head and insufficient header pressure. Bigger is not necessarily better when it comes to surge anticipating relief valves. It is also very important that the sensing line (copper or stainless steel tubing) is routed directly from the header and not from the valve. This ensures accurate header sensing. Surge anticipating relief valves act as an insurance policy by allowing the valve to start opening before a peak on the transient returns. Surge anticipating relief valves can be easily tested and their operation can be replicated in a static condition in the field. This style of valve is very common and reliable.
Conclusion
Transients and surges are a worldwide problem with enormous consequences. Water loss, litigation, and disruption of service and maintenance costs are only a few of the resulting issues. Transient analysis experts are employed by engineering firms around the world, and it is always advisable to consult an expert who can review specific modeling and make informed product selection and decisions. There are many factors to consider when designing water utility distribution systems to ensure good design. There should always be some form of surge or transient protection at every pump station. Diaphragm operated control valves are often a cost effective and practical solution when combined with other sound design practices. Should surge or transient protection be overlooked, resulting loss of power can have devastating effects.
If pumps are utilized in a system (and they are VFD designs), pump control valves are usually not required. If single-speed motors are used on pumps, some form of diaphragm pump control should always be used. Depending on site conditions, booster pump control valves or deep well control valves can be excellent choices. Every pump station requires some form of pressure or transient relief to protect the investment of the pump station and distribution system.
This paper is an excerpt of a white paper released by Singer Valve Inc., and authored by Brad Clarke. To view the full version of the paper, visit www.singervalve.com. ?
Brad Clarke is vice president of sales and marketing for Singer Valve Inc., a global designer and manufacturer of automatic control valves.?
