What is a Backseat?
A backseat is a seating arrangement inside the bonnet.
It provides a seal between the stem and bonnet and prevents system pressure from building against the valve packing when the valve is fully open. Back seats are often applied in gate and globe valves.
Valve Backseat Test Pressure
The test pressure shall not be less than 110% maximum allowable pressure at 38 °C (100 °F). For example, if you have a 900 Ib, 20-inch valve with A216 Gr. WCB material, your MAP will be 2252 psi (as per Table 2-1.1 ASME B16.34). The 110 % of this value will be 2477 psi. So your backseat test pressure will be 2477 psi.
Backseat Test Duration
The valve test duration depends on the valve size as per API Std 598. The test duration is 15 seconds for valve less than 2 inch, 60 seconds for valves greater than 2 inch
The valve packing gland shall be closely inspected while it is under backseat test pressure. Similar to valve shell test, no the leak is acceptable for backseat test.
Why Valves Need Backseat?
When it is opened, the packing is subjected to working pressure. If there is a backseat structure, the working medium pressure can be prevented from acting on the sealing packing, and the life of the sealing packing can be prolonged so that the valve does not have an external leakage.
Another function is that when there is leakage at the sealing packing, the valve can be fully opened to seal the upper sealing portion so that the packing gland or the packing sleeve can be loosened to replace the packing. Therefore, the upper sealing device is required for the gate valve, the globe valve, and the throttle valve.
If the backseat doesn’t leak during the test, then is it acceptable to re-pack a valve on the back seat?
The backseat testing is only done with water at ambient temperature. That does not demonstrate leak tightness for all services the valve could be utilized in, i.e. high-temperature service, hazardous or toxic gases, etc.
A number of the API standards have language that cautions against the indiscriminate use of the backseat. Not all the standards have the same
the language is shown below (i.e. API 6D), but the message here is clear.
API 600, “Steel Gate Valves and Butt-welding Ends, Bolted Bonnets”, section 5.8.9, states “A back seating arrangement is a requirement for all gate and globe valves and, as such, is not meant to imply a manufacturer’s recommendation for its use for the purpose of adding or replacing packing while the valve is under pressure”.
API 589, “Valve Inspection & Testing” section 6.2.3, states “the successful completion of the backseat test shall not be construed as a recommendation
by the valve manufacturer that, while the valve is pressurized, the valve may be repacked or packing may be replaced.”
ZECO Valve is a good 12 inch gate valve supplier, if you are looking for a manual operate gate valve manufacturer, please be free to contact us.
Related Tags :
Ten articles before and after
ANSI B16 Valve Face to Face Dimensions | Zeco
Zeco-Flowturn Sight Glass and Flow Indicator ex stock | Zeco
Improve Total Cost of ownership with valves | Zeco
Proven design, superior quality, price and delivery | Zeco
DOUBLE BLOCK AND BLEED VALVES – DUAL VALVE | Zeco
Compact Quad Rack & Pinnion Actuator | Zeco
