16 replies to this topic

[fallah]

Why is PCV considered as the control valve in min flow line of fire water pumps instead a PV/FV?Is it a NFPA requirement?

Regards

[deltaChe]

The "PCV" you mention is pressure surplusing valve (senses upstream pressure) or a pressure reducing valves (senses downstream pressure)?

[SSWBoy]

In firewater mains you wish to keep the system at a specified pressure (using the jockey pumps) - say, at 10kg/cm2g. If one of the hydrants is used for whatever reason then pressure in the firewater loop will drop and, at a given set point, the main firewater pumps will kick in to maintain the desired system pressure.

[fallah]

PCV senses pressure in its upstream.With thanks to hswang2 and SSWBoy,any other response?

[Zauberberg]

I tend to agree with the Saudi fellow (SSWBoy) because I remember similar arrangement in the plants where I used to work, but perhaps you were asking something different?

Can you upload the P&ID?

[fallah]

I am wondering, even though my question is so clear, why I have not received a direct response. I just want to know "Why is PCV (self regulating valve) considered as the control valve in min flow line of fire water pumps instead a PV/FV?".

The attached document may help for a better understanding.

Regards

Attached Files

•  Fire Water.pdf   53.34KB   229 downloads

[Zauberberg]

You have such arrangement only on the Jockey pumps, P-101 A/B. The Jockey pump is there to kick-in first in case if pressure in the fire main drops below certain value. So, the answer on your question would be - the PCV is not there for the minimum flow, it is there to open and recycle flow to the tank once when the ring main pressure has been re-established. This is my opinion at least, but I don't know all the details since you have copied only a part of P&ID. The entire kick-in/kick-out (startup) logic should be explained somewhere there.

[fallah]

Such an arrangement has been applied for P-102/A/B/C (by PCV 0006) also. Indeed, PCV 0006 and PCV 0007 are minimum flow control valves as per their data sheets. Also, as you know, minimum control valves in pumps are normally closed.

I know the control philosophy of the system exactly, but want to know about applying a PCV instead of a FV/PV.

[Zauberberg]

If there was an FCV, how could we control the fire water system pressure? I don't have experience with fire water systems, but here's my logic:

1. Let's say you open a bleeder somewhere in the fire water system;
2. The ring main pressure starts to drop slowly;
3. Based on the pressure switch settings, the Jockey pump kicks-in and tries to raise the system pressure;
4. Once when the system pressure reaches the PCV set point (which is above the Jockey pump kick-out pressure set point), the valve opens and starts recycling water back to the tank. I believe the PCV is required since there is a minimum run time for the pump, and the pump cannot be simply shut down once when the ring main pressure reaches or exceeds the set point.

I'm looking into our fire water system P&ID and there are no PC valves in the Main Fire Water pump discharge line - just a relief valve. Note on the P&ID says: "The jockey pump controller shall automatically start and stop the pump to maintain the system standby pressure. Kick-in pressure: 9.7barg, kick-out pressure: 10.2barg".

I hope Guido is reading this thread and hopefully he can clarify the subject completely.

[Zauberberg]

In simple words, I see the PCV as a:

- System overpressure protection
- Pump minimum flow provision

But, perhaps I am wrong so let's hope an expert will reply to this thread.

[demank]

Zauberberg's explanation is quite clear for me, because the flow is not important as compared to the pressure regarding the jockey pump.

[fallah]

Thanks a lot for your accurate attention, and using your first point along with my more thoughts i think correspondance of the head and flow in the pump curve causes no major difference between PCV (pressure control leading to have minimum specified flow) and FCV (flow control leading to have maximum specified discharge pressure) considerations. But in system i mentioned, because pressure maintain in the network is the main concern, it has been prefered to use a PCV.

In our system, contrary to your system, the jockey pump is continiously running and obviously needs to be furnished by a minimum flow line (as a consequence of running normally with no flow in order to maintain the pressure).

[Zauberberg]

Thank you for feedback on the subject. I was trying to depict this particular arrangement assuming a flow control valve in the recycle line, and I believe it would be quite difficult to design such a system when knowing that the Jockey pump has to run in between the kick-in/kick-out pressure switch settings, and maintain the minimum flow at the same time. Since there is no control valve in the main discharge line (or any kind of variable resistance device), I wonder what would force the Jockey pump - apart from the ring main pressure itself - to push water into the system and not through the recycle line.

With a PCV, things are somewhat easier in my opinion: as the pressure switch kicks-in the Jockey pump, it starts to pump water into the system, and as the ring main pressure increases, the pump flow decreases due to higher differential head requirements (the pump moves left on the operating curve). At a certain point, which should be the differential head corresponding to the minimum flow, the PCV opens and starts to recycle water back to the tank. That way the pump serves its purpose - maintaining system pressure - while at the same time there is provision for minimum flow.

[fallah]

Thank you for feedback on the subject. I was trying to depict this particular arrangement assuming a flow control valve in the recycle line, and I believe it would be quite difficult to design such a system when knowing that the Jockey pump has to run in between the kick-in/kick-out pressure switch settings, and maintain the minimum flow at the same time. Since there is no control valve in the main discharge line (or any kind of variable resistance device), I wonder what would force the Jockey pump - apart from the ring main pressure itself - to push water into the system and not through the recycle line.

With a PCV, things are somewhat easier in my opinion: as the pressure switch kicks-in the Jockey pump, it starts to pump water into the system, and as the ring main pressure increases, the pump flow decreases due to higher differential head requirements (the pump moves left on the operating curve). At a certain point, which should be the differential head corresponding to the minimum flow, the PCV opens and starts to recycle water back to the tank. That way the pump serves its purpose - maintaining system pressure - while at the same time there is provision for minimum flow.

Agreed with your above nice explanations!

[Zauberberg]

I had a look into the NFPA 20 document and the IPS standard available online, and interestingly no one mentions the Jockey pump recycle line (or maybe I didn't read carefully enough). The IPS standard shows the most common configuration with 2 Main fire pumps and a Jockey pump, the latter one being activated by a pressure switch (not the controller as shown in the sketch), and with a kick-in/kick-out pressure span of 2-3 bar. See attached.

Continuous operation of Jockey pump indicates some significant leakages in your system, in my opinion - unless you use fire water intentionally for some other purpose. So perhaps a quick field investigation can reveal what is all about.

This was very interesting topic, with a few good learning points. Thanks for posting.

Attached Files

•  Fire Systems - IPS.pdf   144.48KB   118 downloads

[fallah]

Continuous operation of Jockey pump indicates some significant leakages in your system, in my opinion - unless you use fire water intentionally for some other purpose. So perhaps a quick field investigation can reveal what is all about.

Good point!

The pipe material of fire water network i attached (as PID) to one of my previous posts,is GRP that as per your above logical statement,because of almost poor mechanical strength tends to be subject to leakage especially on fitting connections.

[kkala]

Unfortunatly I cannot see pdf files (ie the PID) from the summer computer I am using this moment, but following view (subject to comments) may also have some usefulness to this interesting subject. To my understanding the jockey pump operates only for a short time and not during operation of fire water pumps:

Flow control is more precise than pressure control and this is usually applied. But is it more reliable? Flow control requires electronic flow transmitter interlocked with the control valve (through DCS in the majority of cases). I think mechanical interlock is not feasible in such a case; or it would be too complex (as a system), if existed.
Nevertheless mechanical return flow valves (type Yarway or similar) can be found in some critical applications (e.g. liq NH3, boiler feed water). These valves need no electronic parts for action and are usually supplied together with the pump. These are more reliable in comparison to valves controlled electronically, for they can operate even when electronics fail, which is possible during fire.
I assume that these mechanical valves are pressure controlled and they are used on firefighting pumps due to their higher reliability. I do not know whether it is mandatory by NFPA; it may be so, since relevant codes require pump operation at any case, even under power failure (so min flow shall be operable even under electronic failure).