Valves are mechanical devices that control the pressure and flow within a system or process. They are an integral part of a piping system that conveys liquid, gas, vapor, etc. There are many types of valves which are distinctively utilized in different applications, each of which has its unique features and functionality. Some of the common types include: gate valve, butterfly valve, ball valve, pressure relief valve, etc., and today we are going to specifically address the pilot check valves.
Pilot check valves, also referred to as the pilot operated check valves, are normally used to hold pressure in a cylinder in case that there is a drop in pressure or a total loss of pressure. It is performed by allowing free flow from the inlet port through the outlet port. When a pilot pressure is supplied to the pilot port, it allows flow in the opposite direction. Air is able to flow freely as the seal is opened by air pressure on top of the poppet assembly. The poppet checks the trapped air when air flow stops. The trapped air is then allowed to flow back out of the control valve when the valve is piloted.
The pilot operated check valves work by utilizing the cylinder to hold position when the control valve is in its center. When in use, the cylinder tends to creep of the leakage in the control valve. Unlike a basic check valve, a pilot operated check valve can stay open permanently by application of an external pilot pressure signal. A simple check valve cannot be used in such system because bi-directional flow control is required.
There are fundamentally two forms of pilot operated check valves, both of which work in a similar manner compared to a basic check valve, except that the pilot pressure can directly open the valve. One of the forms, the 4C pilot check valve, has the pilot lines connected to the pressure line which feeds the other side of the cylinder. Every time the cylinder moves, one check valve is held open by pilot pressure and the other is held open by flow, which operates as a normal check valve.
Pilot operated check valves have several advantages compared with the conventional check valves. Some of the advantages include:
● Pilot check valves have a smaller package on the larger pipe sizes.
● Pilot check valves offer more options for flow control and pressure.
● Pilot check valves seal more tightly when the system pressure is present, but does not reach set pressure. This makes it a lot less susceptible to leakage.
● Pilot check valves can be installed and mounted remotely.
● Some models of pilot operated check valves enable changes in orifice size with the main valve.
● Pilot check valves typically have a higher operating-to-set pressure ratio.
● Pilot check valves may be used in engines.
● Pilot check valves are hard coated with aluminum with lubricant coating to maximize seal life by reducing wear.
The poppet is designed to distribute and reduce load on the main poppet seal. The poppet is pulled away from the face seal as the pressure increases, allowing wear to be reduced on the poppet and the check valve to operate at higher pressures.
Some of the disadvantages of pilot check valves include:
● In light of the superior functionality, the design of pilot operated check valves is very complex, which often results in various fail-op failure incidents.
● Pilot operated check valves are more expensive than the conventional check valves, even more so when they are at smaller size. It however does start to even out as the pipe size increases.
● Due to the relatively complex parts configuration, pilot check valves are less tolerant to contaminant particles.
● Pilot check valves are typically not capable of handling higher temperature fluids because of the soft goods installed in the valves.
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