Simple steps to help boost the performance of pressure-reducing valves
Pressure management has become a useful tool for controlling leakage in piping networks. This has primarily been achieved because utilities have formed District Metered Areas (DMAs) and pressure zones within their systems. Typically, pressure is managed by using a pressure reducing valve that can either have a single, fixed pressure reducing pilot set point or pressure flow modulating devices to provide a range of set points based on flow.
Whatever the pressure set point, these valves rely on the basic principle of opening and closing a main valve using a pilot system. There is always room for increasing the efficiencies and performance by custom configuring the valves.
Location & Installation
Pilot valves require maintenance, adjustments and electrical connections. Therefore, easy access is important. They cannot be direct buried like gate valves. Typically, they are mounted in valve boxes or underground valve stations in colder climates and above ground where freezing is not a concern.
The valves should be mounted with the stems in a vertical position. They will work in any position, but for practical purposes, install them in a horizontal line, with the stems vertical. This makes future maintenance much easier. Second, leave plenty of space around the valve for future maintenance and access.
Maintaining straight runs before and after the valve is best during installation. The inlet side of the valve is not as critical as the outlet side. A butterfly valve should ideally never be mounted against the valve as this will direct flow in an uneven path into or out of the valve. This is not as critical with gate valves.
The outlet side of the valve is a different matter. Ideally, there should always be a few pipe diameters downstream of the valve to assist in settling the flow after the turbulence of the control valve.
Air in Pipelines
Water contains air, and it can be dissolved in varying amounts, depending on pressure and temperature. At atmospheric pressure, the volume of air varies from 30 percent at 00c to 15 percent at 300c. This dissolved air can come out of solution in a number of ways:
- Water that changes temperature, velocity and pressure, will release air.
- Turbulence caused by rough pipe walls in older mains, bends, valves and other fittings will release air.
- Vortex actions of pumps also cause pockets of air.
Air in a pressure-reducing valve pilot system is an operator’s worst enemy. Compressed air will cause erratic pressure swings and valve modulations that can cause major problems for a pressure management system. Therefore, removing all the air from the pilot system during the commissioning of the valve, or after any valve maintenance, is imperative. Just as important is preventing the air from being introduced into the pilot system in the first place.
Pilot systems and bonnets of control valves form natural high points. These are areas in which any air in a line will collect. Little research has been conducted that addresses air problems in pressure reducing valves, so technical data is limited. Based on 25 years of practical experience, a valve company has found that having an air release valve upstream of the control valve will ensure good, air-free, stable valve operation. Any air in the pipeline is then released before it reaches the control valve. As for sizing, this will depend on flow rates, but typically 25 millimeters (1 inch) will suffice with flows up to 315 liters per second (5,000 gallons per minute).
On the downstream side of the valve, a combination air valve is typically used. A combination air valve is an air vacuum valve that will allow air to vent upon filling the pipeline of the downstream side of the valve and will also allow air to enter when the downstream pipeline is being drained. It has the added advantage of including an air release valve to eliminate any air that may have been formed by the reducing valve as it reduces pressure, allowing even more air to be released. These valves are sized based on flow rate and can get much larger due to the vacuum breaking function of the valve.
If a single valve is installed in a valve chamber or no downstream vacuum protection is required, then the simple inlet release valve is all that is needed.
Combating the Effects of Growth
Valves can certainly be improved in the way that they reduce the friction in the main valve movement. Globally, not all areas have water that is considered pristine. High total dissolved solids (TDS) usually indicate hard water, and this shows itself in systems as scale build up, which with time will do several things:
- Plug the tappings of the main valve sensing ports.
- Grow calcium on surfaces that are situated around theelastomers in the valve. With time, this can puncture the rubber causing failure. (In valves using leather seals, this can be quite catastrophic as the leather will adhere to the cylinder walls and will rip when a valve opens or closes, causing expensive repairs).
- Allow build up on valve stems that can eventually impede the stem movement causing the valve to sit in one position.