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In Piping Plan 74 there is externally supplied barrier gas for Arrangement 3 seals. Barrier gas is maintained at a pressure greater than seal chamber pressure.
Piping Plan 74 systems are used on Arrangement 3, dual pressurized seals, where the barrier medium is a gas. They are the gas barrier equivalent to the traditional Piping Plan 54 liquid barrier system. The most common barrier gas is plant nitrogen. The supply pressure to the seal is typically at least 0.17 Mpa (1.7 bar) (25 psi) greater than the seal chamber pressure. This results in a small amount of gas leakage into the pump, with most of the gas barrier leaking to atmosphere. This arrangement should never be used where the barrier-gas pressure can be less than the sealed pressure. If this were to happen, the entire barrier gas system could become contaminated with the pumped fluid.
Piping Plan 74 systems are typically used in services that are not too hot (within elastomer property limits) but that may contain toxic or hazardous materials whose leakage cannot be tolerated. Because they are pressurized dual seal systems, leakage to the atmosphere is eliminated under normal conditions. Piping Plan 74 may also be used to obtain very high reliability, since solids or other materials that can lead to premature seal failure cannot enter the seal faces. For services containing sticky or polymerizing agents or where dehydration of the pumpage causes solids buildup, Piping Plan 74 systems are not generally recommended.
The Piping Plan 74 system is intended to function as follows:
— from the source supply provided by the plant operator:
— the buffer gas enters the system:;
— mounted on a plate or panel, provided by the seal vendor.
An inlet block valve on the panel is followed by a 2 ym to 3 um filter coalescing filter to remove any particles and liquid that might be present. The gas then flows through a pressure regulator that is set at least 0.17 MPa (1.7 bar) (25 psi) greater than the seal chamber pressure. The flow transmitter with a local indicator follows the regulator and is used to measure the flow and alarm at excessive flow. The next element is a pressure transmitter with a local indicator that is used to confirm adequate pressure. The pressure transmitter is used to raise an alarm upon loss of barrier gas pressure. The last elements on the panel is a check valve. Barrier gas is then routed to the seal using tubing.
No orifice is used in the barrier gas panel to help insure barrier pressure is maintained during high flow rates. For pumps requiring more than one seal assembly (e.g. between bearing pumps), each seal shall be provided with a separate barrier gas panel so failure of one seal assembly will not compromise the performance of the other seal.
- The barrier gas shall be pressurized prior to filling the pump and maintained at all times including standby operation. During standby, small amounts of leakage will continue to leak past the inner seal and collect in the pump. This may require venting the pump prior to starting the pump.