control valve

What is a special purpose valve and what are its types?

A special-purpose valve is a valve that can do certain functions more than the two-way isolation, control, and check. So this type of valve is considered a special purpose valve.

Types of special purpose valves

  1. Multiport valves
  2. Flush bottom valves
  3. Float valves
  4. Foot valves
  5. Line blind valves
  6. Knife gate valves

Multiport valves

A multiport valve is a valve that has more than two ports, these valves are capable to do the operation very quickly and thus the cost can be reduced too. There are three-port, four-port, and five-port valves in the market. The major advantage of a three-way valve is that it can vary the connection of the two supply lines to a common delivery line and flow can be diverted to any of the directions and it can isolate one of the safety valves for the purpose of maintenance. The advantage of a four-way valve is that it can do the reversal of the pump suction and delivery. It can do the bypass, of the strainer or meter and these valves are also capable to do the reversal of flow.

What are the advantages of multi-port valves

  • Fewer valves are needed
  • Fast and reliable operation
  • The piping layout is simplified and thus the economy in fittings
  • The chances of incorrect valve operation are very low, so the risk of product mixing is very less
  • In order to achieve faster operation, two multi-port valves can be inter-coupled

Flush bottom valve

The flush bottom valves are mostly used in the bottom of the reactor vessels, autoclaves, and other process vessels for landing abrasive, slurry, viscous material, or solid media. The disk is in the closed position and it matches with the bottom of the vessel or the piping and thus it won’t leave any room for hold up or stagnation. The flush bottom valves are of two types and they are valves with disk opening into the tank and valves with disk into the valve. In the first case the stem would push the disk into the tank to drain the liquid and this type can’t be used if there is any internal which blocks the disk movement. The draining of the material would be affected. In the second type, the disk would be pulled down into the valve and it would affect the discharge of the material.

Float valves

The major purpose of a float valve is to control the fluid level in a reservoir. The valve inlet will be connected to the supply pipeline and the outlet will be open to the reservoir. There will be a float with the lever and it would control the piston movement and thus the flow is regulated. Mostly these valves would have threaded ends and they are connected to the valve of the reservoir. There is no need to provide a nozzle with the reservoir, only an opening is required and the liquid level will be below the inlet connection.

Foot valves

Foot valves are a type of check valve and they will be placed in the wet wells of the pump. The foot valves have a larger flow area than the actual pipe size so that there will be less head-loss. These valves can be considered as a type of non-return valve and also it would have strainers and these valves are needed when there is a negative suction in the pump.  The priming fluid of the pump can be hold by the check action of the valve. The suction strainer helps to hold the solids while the pump sucks the fluid. Most of the foot valves are designed in a way that, they have self-tapping male and female threads in order to do the installation easily.  

Line blind valves

These valves are used for positive shut-off and replace a spectacle blind, these valves are not like blind plates they are easier to operate and these valves are not expensive. The most common type of line blind valve is the three-bolt/five-bolt line blind valve. There is a single gate line blind valve and this can be power operated or by using a handwheel. A visible wedge line blind valve would provide a full bore opening or positive blinding inside a seat in a three quarter enclosed body. There won’t be any spilling of line residue while changing the wedges and there is no need for line movement when spectacles are changed.

Knife gate valve

These valves can handle dirty or contaminated media and these valves are single seated valves so they can only be used for unidirectional operations. These valves consist of two stationary steel plates and both of them have large holes drilled through them. There is a third plate that slides between the stationary plates in order to perform valve operations. If this is open then it won’t give any resistance to the flow. The major advantage of this design is that the ability of the gate to slip through the plate easily. The gate edge is shaped in a way that it can shear the solids and the elastomer seal would keep the solids away from entering the plates when open.

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