What is a stop check valve?
Stop-check valves are valves used as either shut-off valves or as check valves. This type of valve is made the same as a normal check valve but it has a control mechanism such as a handwheel, lever, or actuator. This control mechanism helps in closing the valve irrespective of the fluid pressure level. Stop check valves are designed similar to globe valves with a long stem. The head of the stem floats on the valve disc. The disc is used to open and close the valve to allow and block fluid flow respectively. The disc is not attached to the stem and acts as a lift check valve where it moves freely up and down to open and close the valve. When fluid attempts to reverse the disc performs as a piston where it closes quickly to prevent backward fluid flow. If necessary, the stem is lowered manually using the handwheel or actuator to stop the valve and thus stop fluid flow. So a stop check valve is a type of valve that is used to open and close fluid flow and is also used as a check valve to automatically prevent reverse fluid flow. Fluid reversing the direction of flow can cause a lot of damage to piping equipment like pumps or boilers as well as cause contamination which could lead to more losses.
Figure: Stop check valve.
Components of a stop check valve
This is the component of a stop check valve that is used to house internal parts of the valve. Stop check valve manufacturers produce the valve body using strong metallic materials to help the valve withstand high pressure, high temperature, and impact damage from falling objects. Some of the materials that can be used to make the valve body are carbon steel, stainless steel, ductile iron, and cast iron among others. The valve body contains the valve ports that connect the stop check valve to the pipeline containing fluid.
This is the top part of the stop check valve which is placed on top of the valve body. The bonnet is used to protect stem sealing as well as other internal parts of the valve. Stop check valve manufacturers connect the bonnet and the body using connections such as bolts and nuts to ensure there is a tight seal free from fluid leakage.
This is the component of a stop check valve which is used to raise and lower the stem to open or close the valve. Some of the stop check valves use an actuator or they use a handwheel. The actuator or handwheel is connected to the valve disc via the valve stem.
This is the component of a stop check valve that connects the actuator or handwheel to the valve disc. The stem serves to transmit the torque from the control mechanism to the disc so that the disc can open or close the fluid flow. Stop check valve manufacturers design the stem with threads so that it can rotate as it moves linearly downwards to block fluid flow regardless of pressure level.
The disc is a component that is used to open or close the fluid flow. Stop check valve manufacturers design the disc such that it can open and close when there is a pressure difference or when fluid attempts to flow backward. The disc can also close regardless of the pressure level by rotating the handwheel to move the stem downwards until the flow is closed completely.
This is the component of a stop check valve that is used to provide a seal between the disc and valve body.
Packing in a stop check valve is a material fixed between the bonnet and the stem to prevent fluid leakage.
Figure: Components of a stop check valve.
How does a stop check valve work?
As mentioned earlier, a stop check valve works as a shut-off valve and also as a check valve. When used as a check valve, it operates on the principle of pressure difference. In this case, the valve opens when the suction pressure is more than the outlet pressure. The incoming fluid should have high-pressure energy to force the valve disc to move upwards allowing passage of the fluid. As a check valve, this valve helps to only allow fluid to flow forward but also closes the valve any time the fluid attempts to reverse the direction of flow. The fluid attempts to flow backward when the suction pressure is lower than the outlet pressure or when the pump is stopped. When the stop check valve is used as a shut-off valve, it closes fluid flow when the handwheel is rotated in the clockwise direction. This rotation makes the stem rotate in the same direction as it moves linearly downwards. This downwards movement of the stem forces the disc to the valve seat and thus helps to close any forward or reverse fluid flow.
Figure: Working of a stop check valve (fully open (left side) and fully closed left side).
Types of stop check valves
Z-shaped stop check valve
This is a stop check which has a z-shape inside the valve housing. This is the most common type of stop check valve. Z-shaped stop check valve is of simple design and thus most preferred. The Z-shaped partition in this type of valve has the seat component. This type of stop check valve has a horizontal seating arrangement where the disc rests. Due to this design, the stem and disc move perpendicular to the pipe axis which results in a high-pressure drop. The valve stem moves up and down through the valve bonnet. The bonnet is connected to the body using bolts and nuts. Stop check valve manufacturers design this valve in a way that results in a valve with a symmetrical shape suitable for easier installation, repair, and maintenance.
Figure: Z-shaped stop check valve.
Y-shaped stop check valve
This is a stop check valve in which the stem and seat are connected at an angle of around 45o degrees as shown below. This type of stop check valve is a suitable alternative to the Z-shaped type as it has a low-pressure loss. This valve can be easily opened for inspection without disconnecting the valve from the pipeline.
Figure: Y-shaped stop check valve.
Angle pattern stop check valve
This valve is a modification of the z-shaped stop check valve. Stop check valve manufacturers design this valve such that the ends of the valve are oriented at an angle of 90o degrees as shown below. According to this stop check valve, the fluid flow takes place at 90o degree angle. Angle pattern stop check valves have the advantage that they can be used as a pipe joint to save on pipe elbow.
Figure: Angle pattern stop check valve.
Flanged end stop check valve
This is a stop check valve that has flanges. Stop check valve manufacturers design the flanges with holes that are used to house bolts used to connect the valve to the pipeline. The pipe also comes with flanged ends to help in aligning and mating the two that is the stop check valve and the pipe. Between the two flanged ends is a gasket placed to help in enhancing a tight seal. These valves are used in high-pressure applications since the torque from each bolt participates in ensuring a very tight seal free from fluid leakage. However, these valves tend to be heavy relative to threaded valves because of the bolts, nuts, and flanged ends used. These valves are easy to open for cleaning, repair and maintenance.
Figure: Flanged end stop check valve.
Threaded end stop check valve
This is a stop check valve that is connected to the pipeline using screws. The stop check valve ports have threads as well as the pipe. If the stop check valve manufacturer produces the valve with female threads, the pipe needs to have male threads. Threaded end stop check valves are easy to install. These valves can also be used in high-pressure applications. Threaded end stop check valves are lighter in weight relative to the flanged end types.
Figure: Threaded end stop check valve.
Applications of stop check valve
- Stop check valves are used in food and beverage production.
- These valves are used in pharmaceutical applications.
- They are used in chemical processing.
- Stop check valves are used in petroleum, oil, and gas refining industries.
- Stop check valves are used in mining applications.
- They are used in wastewater treatment plants.
- These valves are used in the freshwater supply.
- They are used in steam power plants to control the flow of steam.
Advantages of stop check valves
- Stop check valves can be used as shut-off valves.
- These valves operate automatically when used as check valves.
- Stop check valves are durable since they last for long periods. Also, these valves can be repaired when they break down thus enhancing longer service life span.
- These valves are versatile as they can be used in different industrial applications.
- Stop check valves do not allow reverse fluid flow.
- These valves are free from fluid leakage.
- Stop check valves can be used in high pressure and high-temperature applications.
- These valves are easy to install and repair.
- These valves have good shut-off capability.
Disadvantages of stop check valves
- Stop check valves have a high-pressure drop.
- These valves allow fluid flow in one direction only.
- They cannot be inspected while in operation.
Troubleshooting a stop check valve
Packing through the leakage
- The valve packing is loose. Tighten the packing nut with the torque recommended by the stop check valve manufacturer.
- Packing has failed. Replace the packing.
- The packing has insufficient rings. Add more packing rings.
Fluid leakage through sealing surfaces
- Damaged sealing surfaces. Replace the sealing surfaces.
- Dirt on the sealing surfaces. Clean the dirt.
Leakage through the body-bonnet connection
- Loose bonnet bolts and nuts. Tighten the bolts and nuts to the torque recommended by the stop check valve manufacturer.
- Damaged sealing ring or gasket. Replace the metal ring or the gasket as necessary.
The handwheel is not rotating flexibly
- The valve packing is over tightened. Loosen the packing nuts slightly.
- Dirt on the stem. Clean the dirt.
- Stem nut threads are worn out. Replace the stem nut.
The stop check valve does not open fluid flow
- The suction pressure is lower than the outlet pressure. Increase the suction pressure so that the fluid has enough pressure energy to open the valve disc.
A stop check valve is a type of valve that is used as a shut-off valve and also as a check valve. This valve is similar to a globe valve. This valve is meant to prevent reverse fluid flow when used as a check valve. In this case, the valve opens and closes based on the pressure difference. When the inlet fluid pressure is high, it forces the disc to open and thus fluid flows through the valve. When the suction pressure is reduced or when the pump stops, fluid attempts to reverse flow direction and thus the valve disc closes immediately.
To shut off fluid flow through the valve regardless of pressure level, the handwheel is rotated in the clockwise direction. This makes the disc move downwards until it completely closes fluid flow. Having a stop check valve is advantageous in that it does the work of two valves that is as a check valve and as a shut-off valve.
Various types of this valve include Z-shaped stop check valve, angle-pattern stop check valve, and Y-shaped stop check valve, flanged end stop check valve, and threaded end stop check valve. These valves are used in applications such as oil and gas, wastewater treatment, chemicals, pharmaceuticals, foods and beverages, and steam power plants among others. Advantages of using a stop check valve include no reverse fluid flow, durable, versatile, free from fluid leakage, easy to install and repair, and high strength among others.