What is a check valve and how does it operate?

A check valve is a valve that would prevent the flow reversal in a pipeline, these valves are also known as the non-return valves. So by using this valve, we could achieve the unidirectional flow, the input of this valve would be the process fluid. In this valve, it has an arrow in its body to show the proper direction of the flow, we can see this in the below image. So while installing the check valve we must make sure that these valves are mounted properly. This valve would operate according to the pressure of the process fluid and it would only open if the fluid flow is in the correct direction. Valve closure would take place due to the weight of the check mechanism it could be due to the backpressure, by a spring, or by a combination of both. So basically these valves are automatic valves with forwarding flow and it would close in case of the reverse flow.

What are the major types of check valves?

Swing check valve

Major parts of the swing check valve are, the valve body, bonnet, and a disc, and the disc would be attached at the top section of the valve. So if there is no flow the weight of the disc would close the valve. In order to use this valve, we should install this valve in the horizontal pipeline and the disc hinge should be on the top so that gravity would allow to close it. The swing check valve is mostly used where full flow is required. When the fluid comes in the forward direction then the disc would swing in an open position thus allowing the fluid to pass. When the flow of the fluid is in opposite direction then the disc would swing and rest on the seat.

Advantages of swing check valves

• These valves are mostly installed in horizontal lines
• These valves can be used in steam lines
• It can be used with gases and liquids

Lift check valve

These valves are mostly used with air and gas or in case frequent operation of the valve is needed. These valves are mostly used for high-pressure applications where the flow velocity is high. The operation of this valve is entirely dependent on gravity. When the fluid comes in the forward direction then the disc would be lifted from the seat against the gravitational force by the force of the incoming fluid. So the fluid will pass through this direction, If the flow is from the opposite direction then it is according to gravitational force and the disc would remain seated and thus the valve would stay closed. In case the backpressure is low then tight shutoff won’t be possible. There are two types of lift check valves and they are ball check valves and piston check valves.

Advantages of lift check valves

• These valves can be installed in vertical lines, where the flow is normally upward
• Less maintenance
• It can be used for high-pressure service

Ball check valve

In this check valve, there would be a ball in the flow path within the body. The diameter of the ball would be a little bit more than that of the hole. The body has a short length or guide piece in which the ball moves away from the seat to allow fluid passage. In case the pressure behind the seat becomes more than that of the ball then the liquid will flow through. In case the ball pressure is more than the pressure below the seat then there won’t be any flow. In the case of the reverse flow, the ball would reset against the elastomeric seat.

Advantages of ball check valve

These check valves can be used in water condensate lines.

Piston check valve

This type of check valve has a similar operation as the lift check valve, so in this valve, a piston is used in place of a disc. The piston would slide in the cylinder and due to this, there will be a smooth operation. The installation and the flow characteristics of this valve are the same as the lift check valve.

Advantages of piston check valves

• It can be used where there is, frequent variations in the flow direction
• It can be used with water, steam and air system

Tilting disc check valve

In this type of check valve, a disc would be balanced on a pivot, the major parts of this type of check valve are a disc, body, spring, and spring retainer. The disc would move in a plane at right angles to the flow of the fluids, and this movement would be restricted by the spring. If the force which is exerted by the upstream pressure is more than the force which is exerted on the disc by the spring then the disc would lift off. When the differential pressure across the valve is reduced then the spring would force the disc back to the seat thus it would prevent the backflow.  These valves are only suitable for water service because sludge and rags in the dirty fluid would stick onto the disc.

Advantages of tilting disc check valve

• Low head loss
• The opening and closing speed can be varied by using various pneumatic and oil-filled dashpots
• It can be used with air and steam

Diaphragm check valve

In this type of check valve, the closure member is composed of a diaphragm and it would deflect from or against the seat according to the flow. A flexible rubber diaphragm would be used in this valve and it would be placed in the point in the direction of the flow. So if the flow is in the forward direction then it would deflect the diaphragm inwards and thus the fluid would pass. In case there is no flow or there is a back pressure then the diaphragm would return back to its position.

Advantages of diaphragm check valve

• Tight shut-off
• It can be used with slurries
• It is also used in purge gas lines

Butterfly check valve

A butterfly check valve is similar to a butterfly valve, the seating arrangements, and the flow characteristics everything is, similar to the butterfly valve. Mostly these valves are used in conjunction with the butterfly valves. So the operation of this check valve is similar to the butterfly valve. We can install this valve vertically or horizontally with the vertical flow either upward or downward.

Advantages of butterfly check valves

• It can be used with the corrosive media
• It can be installed vertically or horizontally

Stop check valve

We can consider the stop check valve as the combination of the lift check valve and the globe valve. The major part of this valve is the stem and when it is closed then it would prevent the disc from coming off. When the stem is open the valve would act as a lift check valve. These check valves have to override control to stop the flow regardless of the flow direction or pressure. It would close in case of insufficient forward pressure or backflow we can use an external mechanism to quickly close it. This type of check valve can prevent any flow regardless of the forward pressure.

Advantages of stop check valves

• It can stop the flow in both directions
• These valves can prevent any kind of flow regardless of the pressure and direction

What are the factors that must be considered while installing a check valve?

It must be installed far from the pump discharge, we should not use a swing check valve on the reciprocating liquid pumps, we should only use a vertical lift type valve. In case, if there is a water hammer, noise, or shock during the closing of a swing check valve, then it must be replaced by a vertical check valve and while choosing the valve the size must be increased, and thus we could maintain the same pressure drop for the flow.

Where should we use a check valve?

The check valves are used wherever the chances of reverse flow of the process fluid are high. It can be used to protect against two incompatible solutions from mixing, like in a manifold. It is also used in dosing/chemical injection where the high-pressure main must be prevented from flowing back to the smaller chemical feed line.

How is a check valve different from control valves and manual valves?

The operation of the check valve takes place by the process fluid there is no need for any manual or electrical input. So when the fluid flows through the pipeline the flow pressure would allow the check valve to open and in case of reverse flow, it would stay close. So there is no need for electric, pneumatic, or hydraulic actuation which is required for a control valve and also there is no need for operator intervention to open or close the valve, like in a manual valve.

How is a check valve different from a non-return valve?

A non-return valve would allow the flow of the fluid in one direction. A check valve would prevent the backflow in a pipeline, so we can ensure that there won’t be any backflow while using a check valve. This could be very useful in many industrial applications.

Can we consider the check valve as a flow control valve?

A check valve can be described as a directional control valve, it is not used to control the flow we can use this valve to control the direction of process fluid in a pipeline.

How to select a check valve?

• It should be selected according to the closing speed
• Closing characteristics of the valve must be considered
• We should consider the valve size, to determine the flow rate at which the valve is fully open
• It should be used with compressible fluids if it is used with incompressible fluids then the valve would flutter
• Pressure and temperature rating must be considered, we must make sure that the valve is capable to handle the temperature and pressure of the system
• Regulation range
• We must also consider the pressure drop across the valve

What are the advantages of a check valve?

• It can prevent backflow
• It can handle pressure
• These valves can be used as a backup system
• These valves are self actuated, so it doesn’t need an external means to operate
• Fast operation

What are the disadvantages of check valves?

• It can’t be used with the pulsating system
• The closing part of the valve could get damaged
• Valve disc can stick in the open position

What are the applications of check valves?

• Oil and gas
• Petrochemical
• Chemical
• Steam
• Viscous fluids
• It is used for wastewater management
• It is mostly used in pumps and compressor discharge piping