The low surge pilot is typically set at approximately 60 percent of the static system pressure, sufficiently below the system operating pressure. The setting of this pilot is critical, as is the capacity and sizing of the valve. It is imperative that upon operation of this surge-anticipating relief valve that system pressure can recover above this low surge pilot set point so that the valve may close and allow normal operation of the system to resume. If this low surge pilot is set too low or the valve is sized too large, excessive relief flow volume will prevent the system pressure from recovering, since the low surge pilot will not close and so the relief valve will not close, resulting in a complete loss of system pressure and excessive water loss.
Since many relief valves, including surge anticipating relief valves, are often oversized, valves are frequently specified with valve devices designed to limit or restrict the lift or opening under low surge pilot operation. Limiting the valve travel under low surge pilot relief acts to promote the system pressure to recover above the low surge pilot and allow the valve to more reliably close when the surge waves have been dissipated through the open relief valve. These devices are referred to as hydraulic flow limiters (HFL). These provide a compromise to allow the surge anticipating relief valve to be oversized for high pressure relief capacity, yet allow for reliable valve closure under low pressure relief.
Bigger is not necessarily better when it comes to surge anticipating relief valves. It is also very important that the sensing line is connected directly from the header and not from the valve body port, to ensure accurate header pressure 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 are often a good selection when design criteria calls for valves six inches or larger. Surge anticipating relief valves can be easily tested, and their operation can be replicated in a static condition in the field.
Electrically Timed Closure After Power Failure
This valve is similar to the surge anticipating relief valve, with the exception that the low pressure pilot is replaced with a solenoid valve. All other sizing protocol and design criteria remain the same. No static pressure is required as the valve closes on a timer. The valve opens because of either power loss or by way of the high pressure pilot. If power loss is the major concern, then this style of valve can be an excellent selection. Timing for the solenoid to close is normally coordinated with the critical period—the time it takes for the surge return. Always consult with a transient specialist for sizing and selection concerns.
The decision to use, or not to use, surge anticipating relief valves is often based on the perceived complexity of these valves. Having multiple pilots with satisfactory set points, often applied with hydraulic flow limiters and requiring adjustment, along with warnings and dangers of undersized and oversized valves, surge anticipating valves are often misunderstood and overlooked as part of the surge protection in a pump system design.
Rate of Rise Relief Valve
Surge anticipating valves based on the detection of a system pressure abnormality leads to another surge anticipating valve model, the rate of rise relief valve. Valve opening pressure relief on a rapid pressure rise system, even before a high pressure relief setting is reached, anticipates a surge event to allow the valve to respond and open more quickly and efficiently to a surge event.
Rate of rise relief valves add another safety feature to standard relief valves. The valves open when they detect a sudden change in pressure.
On power failure, and after the low pressure interval, the returning wave starts building pressure. The rate of rise pilot senses this rapid increase and immediately opens. This rate of rise surge pilot allows the valve to open, even if only partially, to relieve a greater capacity of the returning surge to minimize the surge below a critical level and of shorter duration. Also equipped with a high surge relief pilot, this pilot will also open fully to ensure the relief valve opens fully, on high surge pressure, but operating from an already partially open relief valve.
This valve model is very attractive, as equipped with a pre-charged accumulator, the valve has positive pressure to force the pilot to open and close. It removes the risk of failure to close, should the pressure not recover. Downstream static pressure is not required for this valve to operate effectively, so downstream elevation is not required.
Another major advantage of the rate of rise relief valve is that sizing is not critical. If the valve is oversized, it will recover and close on conclusion of the transient. It is still recommended that the valve be sized based on roughly 25 percent of the mainline flow rate. The rate of rise relief valve starts opening immediately when pressure begins to rapidly rise. It uses a nitrogen bladder accumulator for very accurate sensing of pressure rise.