Blow down valves
The purpose of blowdown valves is to depressurize the equipment by sending the unwanted gases to flare. Blowdown valves are emergency on-off valves activated by a signal from the Emergency Shut Down (ESD) system, rather than direct activation by overpressure in the protected equipment. Blowdown valves are on-off valves, often installed in a parallel configuration to pressure relief valves on equipment. This depressurization is different than equipment protection performed by pressure relief valves. Certain on-off valves are closed on emergency shutdown (ESD) of a system. Closing of these valves can lead to overpressure in some process equipments. At the same time, ESD system sends signal to open up the blowdown valves and to release the excess gas to flare, thus preventing an overpressure scenario.
Since blowdown valves are simply on-off valves, they cannot restrict the gas flow. The blowdown lines are often equipped with restriction orifices downstream of blowdown valves to restrict the gas flow being sent to the flare to a certain limit.
The term gas blow down is referred to venting of gas accumulated in equipments, process facilities, oil production wells etc. The gas which is to be blown down is not desired to be used for production of a variety of possible reasons and is usually vented through a flare. The flare burns the hydrocarbons before venting product gases to atmosphere.
Gas blowdown in pressure vessels
Emergency shutdown of some processing facilities may end up in sending high pressure gas to certain other pressure vessels, leading to overpressure. To prevent this overpressure scenario, these pressure vessels should be equipped with blowdown valves in addition to the pressure relief valves. Blowdown valves are triggered by the same emergency shutdown to open up and send the excess gas to vent or flare.
Gas blowdown in oil production wells
Oil production wells which have been shut in for a period, can develop a gas cap at the wellbore by accumulation of the gases percolating through the formation. It is desirable to remove this gas cap by flaring the accumulated gases before starting well intervention. This flaring of gases is referred to as casing blowdown.
Other equipments connected to the well to handle the produced fluids are often equipped with blowdown valves as well. These blowdown valves act to prevent the overpressure in case of emergency shoutdown as discussed above.