12 replies to this topic

[vvsastry66]

pLS SEE ATTACHED DRGS.

CO-ORDINATION ERRORS HAVE RESULTED IN INCORPORATING 2 PRESSURE CONTROL VALVES WITH SAME DESIGN CONDITIONS, ON THE SAME FUEL GAS LINE, FIRST ONE CONTROLLING UPSTREAM AND SECOND ONE CONTROLLING DOWNSTREAM PRESSURES.

i WONDER HOW THIS SCHEME WORKS. wILL FINALLY GET A FLOW OF 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM.

APRECIATE ANSWERS.

Attached Files

•  FG PCV.pdf   157.94KB   184 downloads

[Evgeny]

indeed quite strange arrangement.

well, seems by first valve you control pressure in FG header and by second- pressure of FG for users.

I guess you only need to maintain Pressure to users. Therefore you should control it downstream. I would leave only PIC-2.



Regards
Evgeny

[kkala]

As Eugeny points out, CV1 (control valve 1) controls the upstream pressure (37 Barg) and CV2 the downstream pressure (12 Barg). What is the pressure in the intermediate pipe between CV1 and CV2 at FG flow of 1176 kg/h? If close to upstream pressure, CV1 seems redundant; if close to downstream pressure, CV2 seems redundant; in both cases one valve (specified for 37 Barg upstream and 12 Barg downstream) would be considered adequate, receiving action from downstream pressure.

Yet I think plant operator would be reluctant to dismantle one of the valves, probably using following arguments:
1. System of two valves works satisfactorily as is, why touch it? Valve size is small anyway.
2. Action of two valves apparently does not conflict each other; a single valve will control only downstream pressure, not upstream and downstream.
3. If the pair in series is considered as one valve, rangeability would be higher than of the single valve, especially if pressure in intermediate pipe gets values between 37 & 12 Barg (not close to the limits). Variations in FG supply pressure (37 Barg) would be better "faced" by the pair.

At any case I would register openings of CV1, CV2 (as of now) in operation and try to anticipate any maintenance problems (e.g. if too low); and manually set CV1 quite open for some time, to see how the control can be through a single valve.

[djack77494]

There seems to be a lack of appreciation here for the concept of degrees of freedom. The schematic seems to suggest that the two controllers will simultaneously control upstream and downstream pressures. I will unequivocally state that they will not. You have only one independent variable. That is the flowrate of fuel gas from the HP side to the LP side. You cannot have two dependent variables controlled by one independent variable.

[kkala]

There seems to be a lack of appreciation here for the concept of degrees of freedom. The schematic seems to suggest that the two controllers will simultaneously control upstream and downstream pressures. I will unequivocally state that they will not. You have only one independent variable. That is the flowrate of fuel gas from the HP side to the LP side. You cannot have two dependent variables controlled by one independent variable.

My understanding (comments welcomed!) is that there are several consumers (of not stable demand) downstream, tending to lower the 12 Barg pressure. CV2 reacts to maintain pressure by increasing (or decreasing) fuel gas flow. CV1 (accidentally installed) just eliminates pressure variations of upstream fuel gas pressure ( normally 37 Barg) by opening or closing, which affects fuel gas flow.
Resulting intermediate pressure P between CV1 and CV2 is not under control and could render the system uncontrollable in case of high ΔP of CV1, so that P<12 Barg. But the system seems to work, which means that resulting P is adequately above 12 Barg. The two CVs seem to "help" each other. The latter can be looked into by calculation and/or field observation.
Dependent variable to be kept constant is the fuel gas downstream pressure, while fuel gas flow varies to realize it. In this specific case required ΔP (from 37 to 12 Barg) seems possible to be realized in two stages for every flow required. OK, one CV could most probably do the job, but now there are CV1 and CV2 installed.

Edited by kkala, 29 November 2010 - 12:31 PM.

[csp_process]

Newbie here

I agree with Mr. Djack ..

Arrangement is strange indeed..but how is system working fine ? I feel both controllers should interfare with each others proceeding..

[kkala]

Following simplified calculations (hopefully without errors) "immitating" a CV supplier, may be useful.
One valve of max Cv=3.0 (Cv=2.1 at mentioned conditions) could be sufficient for the case (choked flow). Flow regulates downstream pressure, but only upstream pressure affects the Cv size.
Now two valves of max Cv=3.0 have been placed in series. Assume P1=37 Barg at 1176 kg/h, for following three cases:
1α.CV1 remains at set point, not known to us, say at Cv=2.4. Then pressure P downstream CV1 is 29.1 Barg (non choked flow). CV2 has to operate at Cv=2.6 (chocked flow) for P2=12 Barg, which is theoretically possible (but on the high side).
1β. If CV1 set point is at Cv=1.9, results would be P=23.3 Barg, Cv of CV2 = 2.70 (both CVs on non choke flow). Cv=2.70 is theoretically possible (but on the high side).
1γ.Other results Cv of CV1=2.7, P1=37 Barg, P=31 Barg, Cv of CV2=2.5 (chocked flow), case possible.
4th case: Suppose that for case (1β) P1 increases to 41 Barg, when CV1 tends to open so that (assumingly) Cv of CV1=2.7, P=35.6 Barg, Cv of CV2=2.14 (choked flow), case possible.
In conclusion one should know set points and max Cv of the two valves to assess the situation of the "pair" theoretically. Or see it on the field under varying conditions. Control through the "pair" seems impossible, but it is complex needing further study to find advantages, if any.

Edited by kkala, 30 November 2010 - 04:34 PM.

[djack77494]

I don't challenge your notion that the system works, and never meant to imply that it wouldn't. My comment is simply that you are controlling but one independent variable, and that is the downstream pressure. Because you're dealing with a gas, you will have a trapped volume of gas that is free to rise and fall in pressure. It's sort of a buffer, but it is totally unnecessary. You don't care what the pressure of the trapped volume is, right? That pressure is the second independent variable that responds to the actions of the control valves, but who cares what it is. If you wanted to, you could reconfigure the upstream PCV so that it was controlling the pressure in the trapped volume. Then you'd truly have two controllers and two dependent variables. In fact, you might want to do this if the upstream pressure was a lot larger than the downstream pressure, and a single control valve would not be able to adequately control the downstream pressure.

Edited by djack77494, 01 December 2010 - 07:32 AM.

[kkala]

In conclusion one should know set points and max Cv of the two valves to assess the situation of the "pair" theoretically. Or see it on the field under varying conditions. Control through the "pair" seems impossible, but it is complex needing further study to find advantages, if any.

Sorry, apparent error here: Control through the "pair" seems possible, but it is complex needing further study to find advantages, if any.

[Erwin APRIANDI]

I WONDER HOW THIS SCHEME WORKS. wILL FINALLY GET A FLOW OF 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM.

APRECIATE ANSWERS.

I also wonder how this configuration will work, as my understanding is that
the first control valve will control the up stream pressure, means in case of high pressure
is detected by PIC1, it will ask the PCV1 to open more to release pressure
which may increase the flow going in to PCV2 and also the inlet pressure to PCV2 and
PCV2 will react by closing the PCV2 more, so that the downstream pressure maintain

My question to you is "What is the aim of this kind of configuration?", since what I see is, both
of the control valve will just chattering, if the upstream pressure is not stable, and as other has said
the volume of gas inventory between the PCV may be increase (this has to be taken as consideration, based on safety point of view)

And whether this configuration can give you a 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM,
not sure about it, since in case of low pressure is detected by PIC1, it will ask the PCV1 to close more to retain the pressure which may reduce the flow going in to PCV2 and also the inlet pressure to PCV2.
Which I'm trying to said is, the parameter being control here is pressure, means the flow parameter can be changing drasticly depend on the inlet flow.

This is just my understanding, I'm also not very sure about it. Anyone more expert in process control system?

[sheiko]

pLS SEE ATTACHED DRGS.

CO-ORDINATION ERRORS HAVE RESULTED IN INCORPORATING 2 PRESSURE CONTROL VALVES WITH SAME DESIGN CONDITIONS, ON THE SAME FUEL GAS LINE, FIRST ONE CONTROLLING UPSTREAM AND SECOND ONE CONTROLLING DOWNSTREAM PRESSURES.

i WONDER HOW THIS SCHEME WORKS. wILL FINALLY GET A FLOW OF 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM.

APRECIATE ANSWERS.

I don't think there will be any conflict between PV2 and PV1 because the two PCs are not controlling the same region (i.e. the one enclosed by the two PVs).

Edited by sheiko, 12 February 2011 - 09:54 AM.

[Babu Prasad]

pLS SEE ATTACHED DRGS.

CO-ORDINATION ERRORS HAVE RESULTED IN INCORPORATING 2 PRESSURE CONTROL VALVES WITH SAME DESIGN CONDITIONS, ON THE SAME FUEL GAS LINE, FIRST ONE CONTROLLING UPSTREAM AND SECOND ONE CONTROLLING DOWNSTREAM PRESSURES.

i WONDER HOW THIS SCHEME WORKS. wILL FINALLY GET A FLOW OF 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM.

APRECIATE ANSWERS.

This may be designed based on two condition.
1) considering the U/s pressure is main & important rather than fuel gas flow to furnace.
2) this fuel gas supply to furnace May be secondary or backup

[davebartran]

This system works perfectly if the first pressure controller is placed in manual or seriously detuned (minimum gain, no reset).

The usual arrangement is to have both controllers monitoring the down stream pressure of their respective valves.

The real problems arise when you have the fuel demand goes to zero (in your current scheme). When it does the second valve sees the full upstream pressure.

It would be useful to see where the spec breaks occur in your piping, and the over=pressure protection design.

pLS SEE ATTACHED DRGS.

CO-ORDINATION ERRORS HAVE RESULTED IN INCORPORATING 2 PRESSURE CONTROL VALVES WITH SAME DESIGN CONDITIONS, ON THE SAME FUEL GAS LINE, FIRST ONE CONTROLLING UPSTREAM AND SECOND ONE CONTROLLING DOWNSTREAM PRESSURES.

i WONDER HOW THIS SCHEME WORKS. wILL FINALLY GET A FLOW OF 1176 KG/HR AT 12 BARG PRESSURE DOWNSTREAM.

APRECIATE ANSWERS.