The gate valves are so called because the gate-shaped wedge inside is the most used, as well as the most abused, type of valve. Though designed to isolate or connect fluids, they are sometimes wrongly used for flow control. The gate valve should not be used for controlling the fluid flow or for throttling the flow as it can damage the valve due to the high velocity of fluid when the valve is only slightly open. They are normally used in applications where a minimum restriction to the flow of fluid is a design requirement and only an On-Off type of control is required.
Classification of Gate Valves
Gate valves can be classified on the following basis:
- Stem: Whether the stem can rise or is not rising.
- Shape of gate: the gate can be wedge shaped, knife shaped, or of parallel construction.
- Working Pressure
- The distance between the flanges
- Ends: flanged or threaded
Comparison between Gate valve and Globe Valve
- Gate valves are used for On-Off control whereas Globe valves in addition can also be used for the flow regulation.
- Gate valves offer very little resistance to fluid flow in fully open position and also have small pressure drop across the valve. Globe valves on the other hand have a high pressure drop even in fully open conditions and offer substantial resistance to fluid flow.
- Gate valves of the same size are cheaper than globe valves.
- Gate valves because of their design have very little fluid trapping in the line but the globe valves have a larger amount due to the direction of flow.
- Gate valves are unidirectional and can be put around in any way. The globe valves are not.
Use of Gate Valves
The gate valve must be protected in the following ways:
- The valve must be either fully open or fully shut and no intermediate position is allowed.
- The valve should not be over-tightened with the help of long pipes or wheel spanners as distortion of the gate can occur.
- If the valve is getting stuck, it might be due to the deposition of silt and mud in the guides. Slowly opening and closing against the fluid flow many times could clear away the deposits.
- The valve should be fully closed with the hand wheel tight. However when the valve is fully opened, close it a quarter turn to indicate to the other watch keeper that it is open. This same exercise was discussed in the case of globe valves.
- The gate valve can be used in either direction. That means that the suction and the discharge can be reversed.
- The gate valve should not be installed upside down horizontally. As it would encourage the deposit of sediments in the guide ways. However they can be used for vertical duties, but this also should not be encouraged.
Normally the gate valves suffer from one common problem after long service and that is the valve becoming tight due to the deposition of silt on the guide ways. This type of complaint is common and is rectified by opening and cleaning the valve. But in case the valve has been misused and has been used for throttling of the flow, generally wire drawing or erosion cutting of the wedge takes place which must be corrected by replacing the valve, unless we have a spare gate from another old valve. The procedure of overhauling the gate valve is as follows:
- Open the wheel nut and remove the hand wheel.
- Open the gland bolts and slacken the gland flange and the gland bush.
- Open the bonnet bolts and remove the bonnet along with the bonnet and the valve wedge.
- Unscrew the valve stem from beneath the bonnet.
- Clean all the parts by buffing and then blowing with compressed air.
- Examine all the parts for damage and wear. Any marks on the threads must be repaired by dressing the threads.
- The grooves between the wedges seats must be removed by a scraper but be careful not to damage the seats.
- In case the gate or the seats are damaged and it is removable the same can be changed.
- After the cleaning the same can be assembled in the reverse order.
Testing of the Gate Valves
The testing of the valves can be done in a similar way as the globe valves. In brief the valve should be filled up with water and then air should be given from the other side. The presence of air bubbles would indicate bad sealing and must be rectified.