3 replies to this topic

[Rahimzadeh]

Dears::

please tell me why relief valve size of electric driven fire water pump is much less than releif valve of diseil driven pump for same duty service?

 

 

[fallah]

Mr. rahimzadeh,

 

As far as i know in fire water pumping system only diesel driven pumps would be equipped with PSV to protect pump's discharge line against pump's overspeed. The electrical driven pump don't need to be equipped with PSV.

[ankur2061]

Fallah,

 

You are absolutely right for a diesel engine driven pump where overspeed can occur. But NFPA 20 also states that if a variable speed electric motor is used to drive the main fire water pump a relief valve needs to be installed for the same reason i.e overspeed of the motor due to the VFD.

 

NFPA 20 provides the following clause for relief valve installation for fire water centrifugal pumps:

 

5.18.1.1 Where a diesel engine fire pump is installed and where a total of 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated, a pressure relief valve shall be installed.

 

5.18.1.3 Where an electric variable speed pressure limiting control driver is installed, and the maximum total discharge head adjusted for elevation with the pump operating at shutoff and rated speed exceeds the pressure rating of the system components, a pressure relief valve shall be installed.

 

It is important to note here that if the firewater discharge side piping and connected components are full pressure rated for shut-off , the above clauses of NFPA 20 give an option for not providing a relief valve. This is what my understanding is.

 

However, in practice whatever firewater systems I have seen in the field or design documents there definitely is a relief valve on the discharge of the diesel driven firewater pump. However, even for fixed speed electric motor driven pumps I have seen relief valves provided in certain projects where the firewater system vendor has blindly followed the guidelines for providing a relief valve of a given size based on Table 5.25 (a) of NFPA 20. Moreover these relief valves are UL certified. The system vendor thus makes money at the expense of the customer just by quoting that the system is designed as per NFPA standard as required by the client.

 

What a shame that standards can sometimes be misused to make the client pay through his nose to satisfy the greed of the vendor.

 

Regards,

Ankur.

[fallah]

Ankur,

 

Thanks a lot for your clarification. For PSV requirement in fire water pumps i meant it doesn't need if it is fixed speed electric motor driven pump, as you mentioned. Then i am completely agree with you as: VFD electric fire pumps need PSV due to the same reason as diesel engine fire pumps do.