waste cone - waste money ?

[mcarewm]

I just recieved partically completed job that has a fire pump with a waste cone and relief valve designed and spec'd out. Its been years since I have seen one being used . Just wonder what the opinions are ?

Just to add - The pump is 70 psi at 750 gpm and the current flow test is 126,60,1300 (but this test is 100 ft below the bulding) My Pump Churn is well below 175psi with adjustments in elevations .

The pump is already ordered and on site . But next week I am going to RFI the engineer to see why they went with it .

thanks . Happy Holidays

 

[pipesnpumps]

You didn't say, but is the pump electric or engine driven?

If it is electric, then the engineer is probably green, and chose the common method of "check every available option on the cutsheet just in case"... Hey back in my 20's I did the same thing if I couldn't get a senior engineer to help. Of course that was only on preliminary submittals for estimating purposes, not final design.

 

[LCREP]

If it is on a diesel pump or variable speed pump then it is required, see below. If an electric and it does not churn above 175 psi, then it is not required.

4.18 Relief Valves for Centrifugal Pumps.
4.18.1* General.
4.18.1.1 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.

4.18.1.3 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.

****************************************
Fire Sprinklers Save Firefighters’ Lives Too!

 

[mcarewm]

It is in fact an electric , I am going to talk to the engineer after the holidays, . thanks !

If it was a diesel could you just install a pressure relief with out the waste cone ? or do they go hand in hand

 

[pipesnpumps]

How else would you plan on getting rid of the full water flow, if the relief valve lifts?

BTW, don't mis-read LCREP's post. Relief valves are not required on all VSD pumps; only the ones that have a churn pressure higher than 175 psig at full rated speed require this.. If someone manually changes the speed, or the pressure controller/transducer fails, then this protects the piping.

 

[chillylulu]

They are not generally installed on diesel fire pumps anymore, either.

Pressure relief valves and waste cones were very useful years ago, with diesel motors that would race to high rpm's. We also used to use them to pipe the discharge back to the suction, to control the top end pressure (this isn't allowed anymore, but you can still find pumps configured this way.) With modern motors that have reliable governing systems, waste cones are rarely needed. A good design would probably avoid them, as they are a resource (water) waster.

 

[pipesnpumps]

Chillylulu, that is interesting. How does one get around the 4.18.1.1 posted above, which requires them for diesels where 121% of rated speed churn results in >175 psig?

 

[chillylulu]

Modern diesels don't have that problem. The engine rpm's are well regulated and have good governors. The old (20-30 yrs ago +) diesel engines could run to higher rpm's, which necessitated a pressure relief valve and waste cone.

It would be rare to find a listed, diesel driven, fire pump that exceeds the 121% rpm requirement, but I suppose you could order one without a governor.

Regards,

 

[FFP1]

What happens when the diesel engine governor or engine overspeed switch fails to operate as designed???

What has happened can happen & the two scenarios I list above are not uncommon.........this is why the properly sized relief valve (i.e. "waste cone") is required for diesel driven fire pumps. People/engineers/designers who use the "waste cone" approach to prevent excessive system pressures (i.e. >175 psi) need to be more careful and develop better solutions.......the properly sized relief valves or "waste cones" are simply required to ensure the system pressure does not exceed 175 psi in the event of an engine overspeed incident. Because these events are considered "relatively rare events" by NFPA, the discharge side of the relief valves can be piped back to the suction side of the pump.

One more important note.......these relief valves will fail if they are not properly tested and maintained on a regular basis!

 

[b1ueshift]

Is this the circulation relief valve?

Electric pumps are required to have a circulation relief to stop them from overheating if run for long periods of time with zero flow. This needs to discharge to an open drain or an open cone.