Pipe bursts due to transients and surges are common. Not only are the maintenance costs of these repairs high, but when adding possible litigation for third party damages combined with the potential cost of water loss, another pipe burst can have big consequences.
Surges, or transients, are the result of a rapid change in liquid mass velocity within a pipeline. This kinetic energy, released as pressure, acts to expand the pipe diameter and can destroy fittings, pipes, valves, instrumentation and pumps. Pressure waves travel the length of the pipeline of the offending device, then reverse direction. The waves move at a constant speed until they meet a barrier. The reflected and incident waves may superimpose each other to produce a more compounded wave pattern that includes double peaks and double troughs. The consequence of 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 programmable logic controller (PLC) malfunction, single-speed pump motors without adequate pump control valves or the rapid closure of isolation valves.
The selection and sizing of pressure relief valves is very important to protect pipelines, piping equipment and pumps. The application and selection of relief valves may require a detailed analysis by a transient specialist who looks at all factors before making an informed 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. The pressure relief valve is normally mounted off a tee on the header or mainline and opens under high pressure to discharge the relief flow to atmosphere or other suitable low-pressure zone.
It is important to consider the discharge that is released from the relief valves when they react to the overpressure. The discharge can often be chlorinated, which poses a threat for fish-bearing streams, landscaping, etc. A well-considered approach to the proper management of the discharged relief water is always required.
A good initial guideline for sizing a relief valve is to base the flow rate capacity of the valve at 25 percent of the maximum flow in the main pipeline. It is important that the relief valve be sized so as not to be too small or too large. An undersized relief valve will not have enough capacity to relieve the overpressure while an oversized valve will result in excessive flow and possibly a non-recoverable pressure loss.
Pressure relief valves are typically used where there is a risk to the system of higher pressure occurring, and frequently used in systems that have pumps. Their selection and sizing is determined on whether a simple over pressure protection is required, or if there is risk of transients, in which case a surge anticipating relief valve may be selected.
Pressure Relief Valves
A diaphragm actuated, hydraulic pilot operated pressure relief valve responds to opening only when a system is over pressured, typically 10 to 15 percent over the normal operating pressure. If an overpressure or surge pressure occurs, the relief valve opens quickly to dissipate the surge by discharging water to atmosphere. When the system pressure has stabilized, recovering below the surge pressure, the pressure relief valve closes at a controlled rate and normal operation of the system resumes. Rapid or uncontrolled closure of a relief valve may result in secondary surges in the system. Closure rate should not be too fast to create closing surges and not too slow to result in excessive relief water loss.
Surge Anticipating Relief Valves
Standard or single pilot relief valves only open when the system pressure exceeds the relief pilot setting. In a pumping system, where an unexpected pump shutdown occurs, the mass velocity of the water moving away from the pump will initially create an excessively low pressure within the pumping system. This low pressure surge wave will occur prior to an eventual returning high pressure surge wave. The time to open and the capacity of the relief valve opening under this resultant high surge may not be sufficient to adequately dissipate the system surge, resulting in additional and repeated surge waves created within the system. The time, frequency and severity of these waves may become catastrophic and end in a pipe burst or other system failure, even in system where a relief valve was installed and satisfactorily sized. Having the ability to anticipate a surge event would allow the valve to respond and open more quickly and effectively to a surge event.
A dual pilot relief valve, with the addition of a low surge pilot, anticipates the impending high return surge by opening the relief valve upon sensing a low pressure surge. This low surge pilot allows the valve to open and relieve a greater capacity to minimize the surges below a critical level and of shorter duration. The high surge pilot will function normally to allow the relief valve to open fully on high surge pressure, but operating from an already partially open relief valve. Surge anticipating relief valves need significant static pressure to operate properly. A minimum of 43 pounds per square inch (psi) is typical.