gases, pressure drops occur. A bypass valve is installed in the system to continuously regulate the flow pressure and relieve excess pressure in the system. It is used both in centralized networks and in local highways of private houses.

General view of the bypass valve

Purpose and scope

Relief valves are installed in liquid and gas pipelines, in which a regular increase in pressure is possible for various reasons. The task of this device is to maintain the working pressure in the system. With an increase in pressure in the section of the line in front of the installed valve, it dumps part of the working medium into the bypass circuit, thereby reducing the pressure in the main system.

These devices are used in systems:

  • cold and hot water supply,
  • heat supply from any sources,
  • cooling,
  • conditioning.

A separate area is the automotive industry. They are installed in cooling and fuel supply systems. The bypass valves in turbocharged automotive engines control how much air is pumped into the turbocharger.

Automobile engine turbine bypass valve

The design and principle of operation of any bypass valve

Its body is made of steel or brass. The main element of the internal mechanism is a shutter (flap) that closes the through hole. The valve is held closed by a spring. In some models, its role is played by a membrane or diaphragm. The force of the spring is adjusted by a setting lever, brought to the outer surface of the housing.

Valve device diagram


The hydraulics of work is based on the pressure of the flow of the working medium in the pipeline on the valve located inside the housing. As long as the force is less than the lever set by the adjustments, the drain hole remains closed. As soon as the pressure becomes greater than the setting, the pressure on the spring leads to its compression. As a result, the drain hole is ajar, and part of the flow is bypassed into the bypass circuit, reducing the pressure in the main hydraulic system.

Then the reverse process occurs — a decrease in pressure leads to an expansion of the spring and closing of the shutter, and the valve is again ready for the next reset. Pressure equalization occurs continuously, automatically. When the system is operated in the «closed water valve» mode, the bypass channel remains constantly open, providing a constant recirculation of the carrier flow through the bypass circuit.

Cross section of the bypass device

Differences from other types of safety valves

Other valves installed for the safe operation of pipelines have a similar device and principle of operation. But they differ in purpose and requirements.

valve type Mechanism of action Principle of operation
bypass It is installed in the bypass circuit and redirects part of the flow to it Constantly working as needed
Safety Reduces pressure in the system by expelling some of the media out In the event of an emergency increase in pressure
reduction By changing its throughput, it regulates the pressure in the part of the main circuit located after the place of its installation Permanent job

The bypass valve reduces the load on the system pumps without changing the amount of media in it.



The main quantities that determine the possibility of operation of bypass devices in the system:

  • Passage diameter. The internal section of the carrier passage through the valve. It may differ from the diameter of the main contour of the system.
  • throughput. Characterizes the volume of the working medium that can pass through the valve per unit time at a nominal pressure of 1 atm. Measured in cubic meters / hour.
  • Ultimate pressure. The maximum value of the overpressure for which the device is designed to operate. Exceeding this parameter in the system leads to displacement of the valve stem and the beginning of the media bypass. It is determined at a carrier temperature of 20 °C.
  • Setting range. The limits of the overpressure adjustment possibilities at which the valve opens. The unit of measure is bar.
Adjustment scale with adjustment slider

Varieties of bypass valves

With the similarity of the principle of operation and performance, the devices have additional differences.

Connection method

On lines of small diameter (up to 150 mm), the inlet and outlet pipes are usually made for a threaded connection. Options — external or internal thread of the nozzle. On pipelines of large cross section, a connection by welding or using flange connections is used.

Large Diameter Flanged Valves

Flow direction


The valve installed in the main flow is usually angled to connect the outlet circuit. Valves included in the bypass line can also be direct-flow.

Direct flow bypass valve

Active element

Typically, the outlet is closed by a shutter or damper. But in some designs, the locking element is a diaphragm connected to the stem. The use of diaphragm options is recommended in pipelines with a working medium containing solid particles in addition to liquid or gas.

Mechanism of action

The difference in the way the locking element is acted upon led to the emergence of two types of valves.

  • Direct action. A simple mechanical device in which the coolant directly acts on the active element of the valve. Inexpensive and relatively easy to maintain.
  • With elements of indirect influence. In fact, it consists of two valves. The small diameter valve works as a pressure sensor and, when actuated, controls the main valve stem, which opens the drain into the bypass channel. They are characterized by a more precise setting of the threshold.

Device Selection Criteria

Bypass valves are required in the following types of pipelines:

  • Boiler storage systems. The water in them is under pressure, and periodic on / off leads to sharp changes in the volume and pressure of the flowing fluid.
  • Continuous hot water supply systems equipped with temperature control devices. When the temperature changes, the volume of the medium in the line also changes. Constant adjustment and smoothing of pressure drops is required.
  • Multi-circuit heating systems. When any of the circuits is turned off, the pressure in the remaining parts increases. Bypass devices minimize the change in load on the system pumps.
  • Solid fuel heating devices are not able to sharply reduce the temperature of the carrier after shutdown. Transferring the flow to bypass the line allows you to reduce the cooling time.

When choosing a suitable bypass type safety device, the following parameters should be taken into account:

  • Diameter and method of connection, allowing it to be included in a regulated line.
  • The throughput must match the calculated media withdrawal in case of maximum load.
  • The operating temperature of the device and the material of its manufacture.
  • The need to adjust the valve actuation point. Its range should lie within the planned pressure changes.

The focus on the products of a well-known manufacturer is also important.

Mounting Features


The specific installation location of the bypass device depends on the scheme and type of pipeline. The valve can be built into an additional bypass circuit. For heating systems of a closed cycle, excess pressure is discharged into the pipeline in the opposite direction. It can also be used as a safety valve, with an emergency pressure setting and with the withdrawal of liquid into the sewer.

In the scheme of a single-circuit heating main, the bypass valve is installed in the bypass outlet after the pressure pump.

Bypass valve of the local heating system. Installation diagram.

For greater safety and security of the entire heating circuit, it is advisable to install additional ones in addition to the bypass device:

  • check valve to prevent reverse flow,
  • air vent valve to bleed air pockets,
  • drain cock to completely drain the media from the system,
  • for systems of small diameter cottage type — mesh filters.

In multi-circuit systems, by-pass valves are installed in each circuit.

Tips for setup and maintenance

It is worth installing a valve with an overpressure range adjustment for those who already have experience in calculating the required value. The opening of the drain hole starts at the pressure selected by the setting. But it usually opens completely at a pressure exceeding the initial value by 20%. But the calculation cannot be based only on this indicator, because such a decrease in the working head in the system is non-linear. Releasing part of the carrier already leads to a decrease in the load on the valve plug. Therefore, for an accurate calculation of the throughput of the device, they are guided by the diagrams given in the data sheet.

It should be taken into account that the tuning error of most regulators is 10%. For initial adjustment and subsequent control, it is recommended to install pressure gauges before and after the bypass point.

The adjustment itself is carried out either by shifting the slider along the scale, or by turning the adjusting screw. After setting and checking the required value, the screw is fixed with a clamping nut.

Routine care of the bypass valve consists of monthly monitoring of the initial response pressure and its opening speed. You also need to monitor the condition of the filters.

If the valve does not work properly, it should be dismantled, disassembled and washed with all parts. Perhaps the malfunction is caused by a simple clogging of the mechanism.


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