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NFPA 20 - Requirement of Relief Valve and Overspeed of the Engine

Question asked by Kittipong Jongwattanapornchai on May 7, 2018
Latest reply on May 8, 2018 by nocivus

Dear Sir,

 

May I have a question about pressure relief valve installed on the discharge side of the centrifugal fire pump?

 

Clause 4.19.1.2 mentions that relief valve is required if 121% of net rated shutoff pressure plus maximum static suction pressure exceeds the rated pressure of the system. And, clause 11.2.4.4.2 implies that overspeed shutdown of the engine shall be set in the range of 110 to 120 percent of rated engine speed. Hence, the following is a maximum shutoff pressure which can be;

 

Case 1 - Overspeed setting of 110% of rated speed;

Maximum shutoff pressure = 121% of rated shutoff pressure + maximum static suction pressure

Case 2 - Overspeed setting of 120% of rated speed;

Maximum shutoff pressure = 144% of rated shutoff pressure + maximum static suction pressure

 

However, clause A.4.19.1 requires to limit a maximum speed of the engine to 110 percent of rated speed, creating a pressure of 121 percent. The questions are;

 

1. What is a setting of an overspeed shutdown of the engine? 110% or 110-120% of rated speed?

2. If an overspeed of the engine is set at 120% of rated speed, and maximum shutoff pressure of case 1 is less than rated system pressure but maximum shutoff pressure of case 2 exceeds system pressure. Shall relief valve be installed in accordance with NPFA 20?

 

Sincerely yours

Kittipong J.

Rotating Equipment Engineer

PTT Global Chemical PLC.

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