by Rick Bolger
From makeup to process to waste treatment, a well designed backpressure valve with an adjustable set point can be one of the most important tools in a system designer’s arsenal. In most water applications involving chemical treatment, this type of valve is usually constructed of thermoplastic material, and uses a diaphragm to sense upstream pressure and isolate a metal spring. Although it’s commonly referred to as a backpressure valve or relief valve, this workhorse of water treatment also performs the function of a pump bypass valve, backpressure regulator and anti-siphon valve. Most importantly, it can play a vital role in protecting the pump, which is usually the key to the entire piping system.
Plastic relief valves generally feature a two-port or three-port design. While installers and maintenance personnel prefer the convenience of the latter, it’s the former that provides the most multi-function versatility. A two-port design has the capability to completely stop flow below a set pressure point - essential to backpressure and anti-siphon functions. The three-port design is required for bypass and most relief functions, but the two-port valve can also perform these tasks simply when installed on a piping tee.
Let’s take a closer look at each function this one valve offers:
1. Pressure Relief Valve: In this use, the valve protects some part of a piping system - tank, pump, pipe segment, etc. - from excessive pressure. This isn’t to be confused with a pop-safety valve, which must be re-set manually. When the set point of the relief valve is exceeded, the valve opens. When pressure decreases below the set point, the valve closes and the system can function normally again. In Figure 1, a relief valve is installed on a sodium hypochlorite tank in a wastewater treatment facility to prevent overpressure in the tank. The discharge is piped to a sump area.
2. Pump By-Pass Relief: The primary function here is to protect a pump from “dead heading” when there’s an obstruction downstream. The valve opens when pressure exceeds normal levels, sending flow back to a tank or the pump inlet. It effectively creates an “alternate” piping loop for the pump. Considering the expense of replacing a pump and lost downtime, the bypass function is critical. In a groundwater remediation example (see Figure 2), water passes through several different pieces of equipment protected against overpressure via pressure regulators. If one of the regulators closes, flow will stop and the pump will be in danger of dead heading. As the pressure regulator reaches the set pressure and closes, the relief valve senses an increase in system pressure and begins to open, diverting flow back to the suction side of the pump. This is the bypass function. When the pressure regulator opens again, the relief valve senses a decrease in pressure and begins to close, shutting off the bypass and allowing normal flow patterns to resume.
3. Back Pressure Regulator: This is a simple but effective way to retain desired system pressure to points of use. In Figure 3, a kidney dialysis center needs to deliver 0.014 gpm of acidified salt solution and deionized water at 1-8 PSI to 30 points of use, through 950’ of piping. Output at the pump is 32 PSI. A pressure regulator is installed on the front end to regulate pressure downstream, but a backpressure regulator is needed to maintain pressure at the points of use near the end of the system. By installing a diaphragm relief valve set at 5 PSI, it not only assures system pressure will be sustained, but also that it won’t be excessive at the end. The relief valve stays closed to allow pressure to build to 5 PSI, then opens when that’s exceeded. Notice in this application that the same valve is employed ahead of the pressure regulator for the same bypass use described in Figure 2.
4. Pump Back Pressure Valve:In this use, the relief valve provides backpressure directly on the discharge of a pump to enhance pump performance. Here, we have a wastewater treatment facility pumping ferric chloride from a bulk storage tank to a day tank 250’ away. To make sure the metering pump functions smoothly and at maximum efficiency, a relief valve set at 15 psi is installed near the pump. The same valve is also used for pressure relief, as described in Figure 3.
5. Anti-Siphon Valve: Whether it’s an elevation change, or simply the head pressure in a tank, negative pressure and gravity can cause unwanted siphoning. By using a relief valve set at a pressure greater than the head pressure, the valve stays closed until the pump is turned on (when flow is desired) and the set pressure is exceeded. When the pump is turned off and pressure drops, the valve closes and prevents siphoning. In Figure 5, waste treatment ponds at a large dairy operation are downhill from the milking operations. The treatment ponds weren’t functioning properly, and it was discovered the sodium bicarbonate level in the cows’ diet elevated the pH level. A chemical distribution system consisting of sulfuric acid tanks, pH probes and pumps were installed to lower the pH. To prevent siphoning due to elevation drop, a relief valve is used at the outlet of each pump. Valves are set at 10 PSI, which is higher than head pressure from the tanks, yet easily forced open by the pumps. When probes call for sulfuric acid, the pumps turn on and relief valves open, allowing flow of acid to the ponds. When the desired pH level is attained, the pump shuts off and valves close automatically.
You may have noticed that five different liquids are used in the examples above, yet only one valve is employed. Assuming all five piping systems are constructed of PVC, it’s then a question of finding a compatible seal material. Rather than stock a variety of different seal types - and run the risk an operator might install the wrong one in an off-hours emergency - it’s recommended a universal PTFE diaphragm be specified. PTFE is impervious to most chemicals used in water and water treatment. Although any given plant is likely to have a variety of pipe sizes, a universal valve built of universal materials, to meet virtually any backpressure and pump protection needs, has obvious advantages of convenience, efficiency, and safety.
About the Author: Rick Bolger is advertising manager for Plast-O-Matic Valves Inc., of Cedar Grove, NJ. A manufacturer of engineered thermoplastic valves since 1967, it developed the first diaphragm backpressure valve to be state-approved for anti-siphon applications in potable water treatment facilities. Contact: [email protected]