Before determining a suitable method to eliminate or mitigate surging, there are a few things we need to know. When does the surge occur? Is it when a pump starts and stops under normal operating conditions? Or does it occur because the flow is being suddenly shut off, as in a fire hydrant situation? Is it due to a power loss? Or when an isolation valve is closed suddenly downstream of the pump station? These situations are common causes for surging, and some valves can take care of these problems.
You need to know the flow rates and pressure during normal operation for your system. What is the system static pressure? What is the pipe material? How long is the pipeline? What is the pipe size? What is the topography of the pipe?
Caution: If a surge anticipating valve is recommended, then the system will need a discharge for the water that will be released if a surge occurs.
Sizing a Surge Anticipating Valve
First, if the static pressure is higher than 92 feet, a surge anticipating relief valve is one of the best solutions. All surge anticipating valves are sized to operate on momentary condition “M” at 45 feet per second (ft/s) across the seat area when the valve is wide open. This condition occurs as the surge event happens. The surge anticipating valve is sized to operate at 25 percent of the maximum flow rate of the main pipe or header. It will open when the header pressure drops below the lower pilot setting, releasing a transient by keeping the valve open when the header pressure is above the high pilot setting. This valve is hydraulically operated and does not need electrical power.
Second, consider how many pumps are operating at the same time to get the maximum flow rate of the header. This data is needed to size the valve since each pump could handle a different flow. Depending on how many valves are operating, the flow will also be different.
Once the flow across the surge anticipating valve and pressure conditions is determined, use the graph “flow vs. minimum differential pressure” at “M” momentary condition, or check the manufacturers data sheet for the appropriate valve specification.
If the valve is oversized, it will discharge too much pressure to the atmosphere when the surge event occurs.
The static pressure will not recover, and the main valve will remain open until there is no differential pressure across the valve for it to close.
Water loss from an open valve will continue until the main pipe or header is close to empty. When the system recovers, a large amount of energy will be needed to pressurize the header again.
This valve is sized with the same conditions as a surge anticipating relief valve. However, the size is not as critical because it is not affected by the header pressure or oversizing. It is quick to respond and does not need electrical power.
On pipe systems with either a small grade or a flat slope, people tend to think that a transient does not happen. This is not the case.
When a transient occurs, the wave will come back as a big mass of water suddenly increasing the pressure at the header of the pump, which can expand the pipe diameter of the pump system to the point of damaging the pump or worse failure. The surge anticipating on rate of rise of pressure relief valve senses the rise of the pressure and will start to open the main valve to release the overpressure, preventing damages to the pump system.
Do not rely on a valve supplier to do a complete surge analysis. At best, they will be able to calculate the maximum surge you could expect to see over the longest distance of the pipeline, but that does not consider other lines feeding into it or size changes, etc. To accurately predict the surge potential, consider having an engineering company conduct a comprehensive surge analysis on a system.
This can significantly reduce the replacement and repair budget, as well as the headaches associated with damage and interruptions.
If a surge valve size is close to the size of the main line, be cautious. Oversizing of these valves can be as much of a problem as undersizing.
This surge anticipating valve is mounted in a tee, downstream of the pump check valve(s) and a sensing line that is connected from the valve to a connection on the main pump header must be provided.
Tips for Sizing Valves Right
When sizing a valve correctly, users should understand what the valve is required to do and what the system parameters are. This information can allow professionals to pick the valve that will do a proper job while keeping the economic desires in mind. Basically, select the smallest valve that can do the job properly. While making a valve larger than required is a safe choice in many instances, this is not always the case—especially when sizing specialty valves such as surge anticipation valves, where oversizing can be detrimental.
In any valve sizing exercise, there is always an element of choice by the system designer. Is the flow rate given realistic? Are the pressures given accurate and proven? The person responsible for sizing (with the use of graphs, charts and software calculators), who clearly understands the issues, certainly makes for a more educated choice when the time comes to consult with an engineering specialist.