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# Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

## Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

(OP)
NFPA #20, 2013.

Been ages since I've done a diesel engine driven pump so please bear with me.

I have a 1,250 gpm diesel engine driven pump taking suction from a city water main with a flow test of 70 static, 40 residual flowing 1,500 gpm.

ESFR system and I need everything I can get.

Pump looks like it will be rated at 90, 95 or 100 psi and I will make that determination when I see the pump curves as I don't want churn to exceed 175 psi.

Let's assume we have a 95 psi pump with churn at 103 psi. 103 psi+70 psi=173 psi but:

4.18.1.2 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.

(103 psi*1.21=124.7 psi)+70 psi=194.7psi

I am 99% sure that in this case a pressure relief valve would be required, do you agree?

4.18.1.1* Pressure relief valves shall be used only where specifically permitted by this standard.

If I chose a 70 psi pump with a churn of 75 psi a relief valve would be prohibited by the standard.

(75 psi*1.21=90.75 psi)+70 psi=160.8 psi

Come to think of it in 45 years I don't think I've ever installed a diesel taking suction from the city... it's always been diesel engine driven vertical turbine pumps taking suction from a pond of which there's been at least half a dozen.

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

Yes. You will need a Pressure Relief Valve based on your first example.

We do a few of these every year and end up with a relief valve on the diesel pumps. We still don't get the churn above system component ratings. But, that 121% above churn will get you many times.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC
www.mfpdesign.com

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

(OP)
Thanks Travis, that is exactly how I interpreted it but just wanted to make sure.

And, again to make sure, on the second example where I am 160.8 psi I am prohibited from installing a relief valve, right?

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

Correct. If your churn *1.21 is <175 then you can't do a pressure relief valve. Plus, you wouldn't need one.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC
www.mfpdesign.com

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

The relief valve will dump a LOT of water during the 30 min weekly churn test. Hopefully you have a place to drain it to, if not it may give the occupants an excuse not to run the pump.

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

(OP)
If I am interpreting it correctly, and I do think I am, unless you have a malfunction the relief valve should never discharge water.

In my specific case we have a 70 psi static with a 95 psi pump with churn at 103 psi. 103 psi+70 psi=173 psi.

Personally I would be surprised to ever see a suction pressure, at the pump suction flange, greater than say 67 psi because 1)the pump sits approximately 1'-6" above the hydrant butt where the static and residual pressures were taken (.7 psi but I will take it) and 2) we have an 8" Ames 3000ss double detector check and at churn we're going to be losing maybe 2.5 psi through the detector check.

Would I rely on the detector check head loss? Not sure how accurate at little to no flow the chart is but experience in doing pump tests tells me to expect a few psi head loss through the detector check at churn. Next time I am at a pump test I think I will fix two certified gauges upstream and downstream the detector check just to see if my hunch of losing 2 to 3 psi is justified.

I bet our churn turns out to be 168 psi and if set properly the relief valve should never discharge unless we have a runaway pump. The whole reason for a relief valve.

I never did it but years ago I do believe some designers would oversize a pump but control the discharge pressure by using a relief valve which always seems a little silly to me but since those days it appears they changed to standard so we can no longer do that.

The relief valve will discharge into an open ditch draining to a retention pond.

### RE: Diesel engine driven pumps and pressure relief valves - I think I have to have one, am I correct?

Is the 70 psi static the highest you get from seasonal fluctuations? Could it go higher?

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