A back pressure valve is an automatic valve that keeps the pressure on the primary side (inlet side) of the valve near a set value by releasing the excess fluid to the secondary side as the pressure rises. It opens when the inlet-side pressure exceeds the setting and closes when it falls, thereby maintaining the upstream line pressure within a fixed range. This is the opposite of a pressure reducing valve, which keeps the secondary-side (outlet-side) pressure constant: both the “side being watched” and the “purpose” are reversed, and confusing this difference leads to selection errors. In conclusion, the basic distinction is to use a back pressure valve when you want to stabilize the upstream-side pressure, and a pressure reducing valve when you want to stabilize the pressure supplied downstream.
Key Points of This Article
- A back pressure valve keeps the primary-side (inlet) pressure constant, while a pressure reducing valve keeps the secondary-side (outlet) pressure constant; the object of control is reversed.
- A back pressure valve opens when the inlet pressure exceeds the setting, releasing the surplus fluid to stabilize the pressure of the pump and upstream equipment.
- Its purpose differs from that of a safety valve: the back pressure valve handles “continuous pressure regulation,” while the safety valve handles “pressure relief during abnormal conditions.”
The Role of a Back Pressure Valve and How It Works
Operation That Keeps the Primary-Side Pressure Constant
A back pressure valve operates by balancing the primary-side pressure acting on the valve body against the set force of a spring (or a mechanism combined with a diaphragm). When the inlet-side pressure exceeds the set value, the valve opens and lets the excess fluid flow to the secondary side, lowering the pressure. Conversely, when the inlet pressure falls below the setting, the valve closes and builds up pressure on the upstream side. Through this repeated action, it keeps the upstream pressure of a pump discharge line or a circulation piping system within a fixed range.
Typical applications include stabilizing the pump discharge pressure to suppress flow rate fluctuations, distributing fluid evenly among multiple branches, and securing a constant back pressure to keep downstream equipment operating stably.
Difference from a Pressure Reducing Valve
The most easily confused point is the difference from a pressure reducing valve. A pressure reducing valve detects the secondary-side (outlet-side) pressure and automatically adjusts the valve opening to keep it at the set value. In other words, a pressure reducing valve “keeps the pressure sent downstream constant.”
A back pressure valve, on the other hand, detects the primary-side (inlet-side) pressure and keeps it constant. A back pressure valve “keeps the upstream-side pressure constant.” Although both control pressure, the side they use as their reference is exactly opposite, so you must select the valve by working backward from the function you need. The starting point for selection is to first clarify whether you want to stabilize the supply side or the supply source (upstream).
Positioning Relative to Safety Valves and Relief Valves
A back pressure valve can resemble a safety valve or relief valve in structure and appearance, but its purpose differs. A safety valve is normally closed and opens only when an abnormal pressure rise occurs, protecting the equipment; it is an “emergency” device.
By contrast, a back pressure valve is positioned as a “control” valve that continuously regulates pressure within normal operating conditions. The valve you should choose changes depending on whether the required role is “protection during abnormal conditions” or “pressure stabilization during normal operation.” Depending on the equipment requirements, the two may also be used together.
Frequently Asked Questions
Q1. Which should I choose, a back pressure valve or a pressure reducing valve?
A1. As a basic rule, choose a pressure reducing valve if you want to stabilize the pressure sent downstream, and a back pressure valve if you want to stabilize the upstream (supply source) pressure. Decide based on which side you want to control.
Q2. Can a back pressure valve also control flow rate?
A2. Its main purpose is to stabilize pressure; it is not a device that directly keeps the flow rate constant. Flow rate fluctuations may be suppressed as a result, but flow rate management requires separate consideration.
Q3. How is the set pressure determined?
A3. It is generally determined based on the allowable pressure of upstream equipment and the pressure required downstream, but it varies with conditions. Because the fluid used, temperature, and bore size also have an effect, it is safer to confirm it based on the actual operating conditions.
Q4. Will a back pressure valve also keep the secondary-side pressure constant?
A4. In general, what a back pressure valve maintains is the primary-side pressure. If you want to keep the secondary-side pressure constant, a pressure reducing valve is suitable. Please consider the valve type according to your purpose.
Q5. Can it be used even with different fluids such as steam, water, or air?
A5. You need to select a product with specifications suited to the fluid and to the temperature and pressure conditions. Even among “back pressure valves,” the material, seals, and compatible fluids differ, so organize your conditions before making a selection.
Q6. I want to replace an existing back pressure valve with an equivalent product.
A6. Selection is made after confirming the bore size, connection type, set pressure range, and fluid used. Knowing the nameplate information and piping conditions makes it easier to narrow down an equivalent product.
Summary
A back pressure valve is a control valve that keeps the primary-side pressure constant, and its object of control is the reverse of a pressure reducing valve, which maintains the secondary side. Because its purpose also differs from that of a similarly shaped safety valve, the shortcut to avoiding mistakes is to select it around the question of “which side’s pressure do you want to stabilize during normal operation?”
In making a selection, you need to make a comprehensive judgment based on multiple conditions such as the fluid used, temperature, pressure, bore size, and connection type. It is perfectly fine even if you are still at a stage such as “I have not sorted out whether I want to stabilize the upstream or the downstream,” “I know the maximum flow rate but not the minimum flow rate,” “I cannot decide on a target set pressure,” or “I want to select an equivalent product for an existing valve.” Even when your conditions are not yet fixed, please feel free to contact Yoshitake for technical and selection consultations. We will help narrow down the suitable valve type and specifications while learning about your operating conditions.