Tuesday, September 3, 2013

An Alternative to Safety Relief Valves - Safety Trip Stop Valve

Figure 1
The Safety Trip Stop Valve shown in figure 1 is an example of the automated valves supplied by RENCOR Controls as an alternative to Safety Relief Valves as allowed by the Power Piping Code 1987Adenda to the 1986 issue. B31.1 paragraph 122.14.12.

The Power Piping Code allows for the use of this Safety Stop Valve as an alternative to Safety Relief Valves in applications where their use is difficult or impractical. Paragraph 122.14.12 from the Power Piping Code B31.1 reads as follows:


122.14.2 Alternative Systems. In district heating and steam distribution systems where the steam pressure does not exceed 400 psi (2750 kPa) and where the use of relief valves as described in Para. 122.14.1 is not feasible (e.g., because there is no acceptable discharge location for the vent piping), alternative designs may be substituted for the relief devices. In either case, it is recommended that alarms be provided which will reliably warn the operator of failure of any pressure reducing valve.

(A) Tandem Steam Pressure Reducing Valves. Two or more steam pressure reducing valves capable of independent operation may be installed in series, each set at or below the safe working pressure of the equipment and piping system served. In this case, no relief device is required.

Each pressure reducing valve shall have the capability of closing off against full line pressure, and of controlling the reduced pressure at or below the design pressure of the low pressure system, in the event that the other valve fails open.

(B) Trip Stop Valves. A trip stop steam valve set to close at or below the design pressure of the low pressure system may be used in place of a second reducing valve or a relief valve.

122.14.2 Alternative Systems. In district heating and steam distribution systems where the steam pressure does not exceed 400 psi (2750 kPa) and where the use of relief valves as described in Para. 122.14.1 is not feasible (e.g., because there is no acceptable discharge location for the vent piping), alternative designs may be substituted for the relief devices. In either case, it is recommended that alarms be provided which will reliably warn the operator of failure of any pressure reducing valve.

(A) Tandem Steam Pressure Reducing Valves. Two or more steam pressure reducing valves capable of independent operation may be installed in series, each set at or below the safe working pressure of the equipment and piping system served. In this case, no relief device is required.

Each pressure reducing valve shall have the capability of closing off against full line pressure, and of controlling the reduced pressure at or below the design pressure of the low pressure system, in the event that the other valve fails open.

(B) Trip Stop Valves. A trip stop steam valve set to close at or below the design pressure of the low pressure system may be used in place of a second reducing valve or a relief valve.

The use of the RENCOR Safety Trip Stop Valve allows for compliance at a lower equipment cost than the traditional methods. The bigger savings results from the significantly less expensive purchase price and more importantly, installation cost.

The tight closing metal seating system allows for use in super heated steam up to 1000 F and also eliminates any chance for high velocity erosion when closing. The port of the valve is full bore from flange to flange so that when the valve is in the normally open position there is no pressure drop greater than that of an equivalent length of pipe. Also, erosion will less than that of an equivalent length of pipe due to the smooth Stainless Steel internals.

OPERATION:

RENCOR’s Safety Trip Stop Valve can be installed in a steam line, usually up stream of the consumer, Pressure Reducing Valve or a Steam Conditioning Valve. A pressure sensor is mounted downstream of the PRV or Steam Conditioning Valve, with a set point as required by B31.1 as described above. In the event of a pressure increase at or above the set point the sensor signals the trip mechanism and the valve snaps closed, driven by a large spring in the actuator. An alarm can be incorporated in the trip action to alert an operator in the control room and else where.

In order to return to service, it is necessary for an operator to locate the valve and manually reset it. This provides for additional safety by forcing an opportunity for inspection of the installation before proceeding with resumption of operations.