4 replies to this topic

[Roark]

I am commissioning the firewater system of a large facility and I've come across an issue in our jockey pumps: our C&Es state that if the pressure in the firewater header drops below a certain point (e.g. 150 psig) the jockey pump shall run for a predetermined period of time (30s in this case). If the pressure is still below the set point after that period of time, the pump will run for periods of 30s until the set point is attained. This suggests the use of a pressure level switch without a deadband, i.e. with only ONE set point.

However, on the field I've realised that our jockey pump controller has a timer (which is OK as per our C&Es) and that the pressure switch has a minimum deadband of 12 psi. I do understand that a strategy to control the pressure in a firewater header is using LO and HI setpoints (i.e. a deadband), but to me, it doesn't make sense using a timer AND two setpoints, as our system installed in the field seems to use.

Is this a standard practice?

Also, on a related note, our jockey pumps use a sensing line on the discharge line of the pump (as per NFPA 20), which is a 1.5in pipe. After installing a pressure gage in this sensing line we saw that the static pressure when there's no flow (i.e. pressure in the ringmain) was, say, 140 psig, but after the pump cut-in it increased some 15 psi. I imagine this may be due to friction losses in the discharge line. However, it seems that an NFPA20 fully compliant design implies that the pressure drop in the discharge line of the jockey pump must be negligible because it assumes that the pressure in the discharge line is the same as in the ringmain (i.e. it uses the discharge pressure to stop the pump).

Are my observations correct? Is it acceptable to have a jockey pump design with a relatively high pressure drop in the discharge line?

Any feedback/discussion will be appreciated.

[VikingUK]

The timer is there in case the pressure sensors fail. ie it will at least run for 30 secs and so maintain pressure. This is probably because the ESD system will kick in on low ring main pressure eg if someone is using a fire hose in an emergency or a deluge has fired. You don't want the ESD system to kick in everytime you get a slight decay in ring main pressure.

[fallah]

I am commissioning the firewater system of a large facility and I've come across an issue in our jockey pumps: our C&Es state that if the pressure in the firewater header drops below a certain point (e.g. 150 psig) the jockey pump shall run for a predetermined period of time (30s in this case). If the pressure is still below the set point after that period of time, the pump will run for periods of 30s until the set point is attained.

As far as I know,what you mentioned is about main firewater pumps which the first one starts if the main ring pressure falls below its set point. If the main ring pressure is still below its set point after a suitable delay (20-30 s) the second pump starts automatically.

There is another story about jocky pumps.One jocky pump shall run continuously,the spare shall be started automatically in case of fault of the first one.

[akslzf]

The 30 sec timer, as per your explanation, sounds unusual!!

It is most likely something like this: If the required header pressure is 150 psiG and the jockey pump kick-in pressure is 140 psiG, the pressure switch fires the jockey pump ONLY if the header pressure is <140 psiG for a CONTINUOUS 30 SEC. Likewise, once the header pressure hits 150 psiG, the trigger waits for 30 sec before putting off the jockey.

This is, so as to avoid any transient pressure spikes in the header due to start / stop of pumps or activating a fire monitor. it is for the same reason that the pressure switch is usually installed at about 25-50m down stream of the pumps' discharge.

Since operation of jockey pumps is intermittent, take abundant care in sizing the recycle line. Also provide a simple self-venting (ball-float type) valve on the discharge line.

If the header is in steady state (no leak, no fire monitors open & no pumps running), the static header pressure, as the name implies, must be static (say 150 psiG) throughout!! Remember Pascal's law?? It is only when there's flow, there's friction!!

[chemtan]

Dear,

For the timer on Pump start and on set point achievement, it seems like it is based on pump protection philosophy. In both cases (pump start and stop), the intention is to provide some time to prevent a situation where pump starts and stops immediately and then restart. Load on cables at pump startup are quite big and such a scenario should be avoided where possible - an electrical engineer can elaborate that better.

As for the sensing line issue - I failed to understand what is the problem that you have there. We know that pressure in liquids is transmitted equally in closed systems (also noted by another reply in this post).