18.0 CHECK VALVES
The check valve has a specific purpose, that is, to protect plant and equipment from the consequences of reversed flow conditions. If the flow does reverse, the pressure of the fluid will help to keep the valve closed.
Check valves are usually installed in pump discharge lines to prevent back pressure from returning fluid through the pump to a vessel or tank. They may also be found on wellhead assemblies upstream or downstream of a choke valve. Although check valves are useful where fluctuations of pressure occur, do not depend on them for total isolation if a pump fails. Always shut off the nearest relevant manually operated valve. A check is only a temporary safety device. A check valve is not an isolation valve. Check or non return valves can be classified according to there mode of action into the following types.
- Flap
- Ball
- Lift
- Swing
- Split
- Piston or Spring
18.1 Flap Check Valve
A flap check valve has hinged disc which is raised by the fluid passing through the pipe. Low pressure fittings have an inspection cap provided. Normal flow lifts the flap but a reversed flow will press the flap hard against its seat, closing the valve. When the upstream pressure returns to normal, the flap will be lifted again to allow flow. Flap valves are used in horizontal flow situations.
18.2 Ball Check Valve
This valve must not be confused with the ball shut-off valve. The ball check valve can be used in either horizontal or vertical flow situations. The design for each type is different. The horizontal flow ball check valve has a stepped barrier with a central hole at 90º to the pipe bore. A free moving ball sits in the hole and is raised by the upstream flow pressure. In the vertical flow ball check valve the upstream flow pressure raises the ball above its seat. If reverse flow occurs, the downstream pressure will force the ball hard against its seat to cut off the reverse flow of fluid.
18.3 Lift Check Valve
The operation of the lift check valve is similar to the ball check valve but a plunger type disc is used to close the valve instead of a ball. The horizontal lift check valve has a stepped barrier with a central hole at 90º to the pipe bore. A free moving plunger sits in the hole and is raised by upstream flow pressure.
In the vertical lift check valve the upstream flow pressure raises the plunger above its seat. If reverse flow occurs the downstream pressure will force the plunger hard against its seat to cut off the reverse flow of fluid.
FIGURE 18.3
LIFT CHECK VALVE
This type of check valve is potentially fast closing because the lift of the valve is small. If dirt enters the guide in which the closure member moves, the closure member may stick or close late. Viscous fluids make the closure member in its guide sluggish.
18.4 Swing (Y) Check Valve
The swing check valve is similar to the flap check valve. It differs in that the hinged disc is angled at about 45º to the pipe bore. The swing check valve can only be used in a horizontal position. The upstream flow pressure opens the hinged disc to allow flow. If reverse flow occurs the downstream pressure closes the disc and cuts off the reverse flow.
18.5 Split Check Valve
This valve is constructed with two semicircular spring loaded discs attached to a central pivot pin. The spring keeps the discs in a normally closed position.
Upstream flow pressure forces the two discs to open until they are parallel to the flow direction. Reverse pressure will close the spring assisted discs to prevent a flow of fluid.
FIGURE 18.4
SPLIT CHECK VALVE
18.6 Piston Check Valve
The piston check valve consists of a housing or body containing a moving piston pressed downwards on its seat by means of a spring. The valve is in the open position when the pressure underneath the piston is higher than the spring tension, lifting the valve from its seat. The valve is closed automatically when this pressure is lower, for example, by a pump failure. The back pressure from the system will then support the spring pressure. The piston check valve closes much faster than the hinged disc or ball type non-return valves, and is commonly installed on compressor discharge lines. It is designed to handle fluids of high temperatures (up to 750ºF) and pressures (up to 2000 psig).
FIGURE 18.5
PISTON TYPE WITH MANUAL HAND WHEEL
Some of the desirable features of a non return valve include:
- Slam free closure
- Tight closure – does not pass
- Minimum pressure drop – saving of energy
- Maintenance free
- Suitable for all liquids and gases
- Quick acting
- Installation in varying positions – small size
- Wide pressure range
- 7 Gauge Cock Valve
- Figure 18 illustrates a gauge cock valve. This is a special type of valve. It is used between a vessel and a vessel gauge glass.
- FIGURE 18.6
- GAUGE COCK VALVE
- Its function is to isolate the gauge glass automatically if the gauge glass begins to leak or break.
The gauge cock valve has a small metallic ball inside. If the gauge glass breaks, the fluid inside the vessel will flow out through the valve and out of the gauge glass. The fluid movement pushes the metal ball into the outlet hole of the valve. This closes the valve and so stops the leak.
A gauge cock valve must always be in the fully open position when it is in service or it cannot protect the system. If it is only part open the valve stem will stop the ball from sealing off the outlet hole.
If the gauge glass must be isolated for cleaning etc. the gauge valve can be manually operated.