14 replies to this topic

[lakshmi2011]

I have an offshore fire water pump which is rated for 1218m³/hr at 12 barg.After some additions in the Offshore platform, the total demand increased to 1495 m³/hr and pressure at pump discharge is 11 barg.

Does this pump still meet NFPA 20 requirement of "pump shall discharge atleast 150% of rated capacity atleast 65% of rated head".How do I determine this ?

I have the fire water pump curve with me but do we still determine 150 % based on initial rated capacity or the increased demand .

How do we typically cater to increasing FW demand ?

Can anyone help me in clearly understanding the problem?

[kkala]

I have an offshore fire water pump which is rated for 1218m³/hr at 12 barg.After some additions in the Offshore platform, the total demand increased to 1495 m³/hr and pressure at pump discharge is 11 barg.

Does this pump still meet NFPA 20 requirement of "pump shall discharge atleast 150% of rated capacity atleast 65% of rated head".How do I determine this ?I have the fire water [color=red pump[/color] curve with me but do we still determine 150 % based on initial rated capacity or the increased demand .How do we typically cater to increasing FW demand ? Can anyone help me in clearly understanding the problem?

Not being experienced on the topic, I can only tell my opinion.
1. The new data of "1495 m3/h at 11 bar head" covers the revised design conditions, so NFPA requirement becomes "at least 2243 m3/h at 7.15 bar head". Or "at least 7.15 bar head at 2243 m3/h flow", if this facilitates reading of the pump curve diagram.
2. You have to verify through existing pump curve whether above mentioned condition is satisfied. Chances are that it is not. For instance, both pump curves in the attached "FFpump.xls" satisfy original data of "1218 m3/h at head=12 bar","1495 m3/h at head=11 bar", as well as "at least 7.8 Bar head at 1827 m3/h"; but only the blue curve (not the pink) satisfies the requirement "at least 7.15 bar head at 2243 m3/h".
Probably (but not quite) supplier of the fire water pumps had placed a generous margin in the past, its adequacy has to be checked now. Let us know what the pump curve indicates at 2243 m3/h, if you can.

Attached Files

•  FFpump.xls   15.5KB   379 downloads

[lakshmi2011]

So does it mean, 150% changes based on Fire water demand or is this criteria required only to be met when the pump is first commissioned?


what I mean to clarify is NPFA 20 criteria for 150% of rated capacity atleast 65% of rated head is applicable is based on Rated head (which was specified when the pump was purchased) or actual changed demand ?

[kkala]

What I mean to clarify is : NPFA 20 criteria for 150% of rated capacity at 65% of rated head is applicable on Rated head (which was specified when the pump was purchased) or actual changed demand ?
Without being aware of NFPA 20, I believe that mentioned requirement cannot be based on original data that has been changed (to worse), but on 1495 m3/h at 11 bar head, as explained on the post of 30 Jul 2011.
Note: The revised pump data is expected to have come out of a relevant study; the new data specifies rated flow and head now and had better be verified by test on field (I do not know whether test is mandatory by NFPA, if you have performed the original test; chances are that it is, search NFPA for it).

Edited by kkala, 17 August 2011 - 12:00 PM.

[lakshmi2011]

Thank you very much for the reply.

The new data came up as a result of revised firewater demand and hydraulic calculations.

if the specific pump is able to meet the required demand with some modifications like adding Restriction orifices , do we still need to change the pump just to satisfy NFPA requirement?

What I mean to clarify is : NPFA 20 criteria for 150% of rated capacity at 65% of rated head is applicable on Rated head (which was specified when the pump was purchased) or actual changed demand ?
Without being aware of NFPA 20, I believe that mentioned requirement cannot be based on original data that has been changed (to worse), but on 1495 m3/h at 11 bar head, as explained on the post of 30 Jul 2011.
Note: The revised pump data is expected to have come out of a relevant study; the new data specifies rated flow and head now and had better be verified by test on field (I do not know whether test is mandatory by NFPA, if you have performed the original test; chances are that it is, search NFPA for it).

[kkala]

...
The new data came up as a result of revised firewater demand and hydraulic calculations.
if the specific pump is able to meet the required demand with some modifications like adding Restriction orifices , do we still need to change the pump just to satisfy NFPA requirement?

Why place restriction orifice if not needed? This will cause some decrease in flow. If the pump delivers more than 1495 m3/h, never mind.
Suppose that the pump can transfer 1495 m3/h at 11 bar head, but not the extra NFPA requirement of 2243 m3/h at 7.15 bar head. Probably you have to ask advise from local fire police authority. They check and approve the fire fighting design and installations, so they have the last word. You can look into local regulations too.

[breizh]

Good day all,
You can dowload from internet an old version of NFPA 20 , this will give you more information :

http://www.minhbao.c...le/A_NFPA20.pdf


For insurance purpose you need to make sure your pump is doing the job otherwise you will get trouble !

You need to perform test regularly ( DP vs flow) to ensure your system is in good condition .


My understanding

Breizh

[S.AHMAD]

Dear Lakshmi,

The NFPA20 requirements must be based on the revised rated capacity. The 150% and 65% for capacity and head respectively are the safety margins for firewater pump - if you do not comply to this that means you do not have the required safety margins as per the standard requirement. Regardless of whether it is for new pump or for an existing pump.

If the existing pump cannot meet the NFPA20 requirement, then you can modify for example by increasing the impeller size (if it is still not maximum size) or better still buy a new pump. However, please go through in more details of the standards whether two pumps in parallel is acceptable for meeting the standards. If it does not mention about it then I believe that installing two pumps in parallel is acceptable.

Installing orifice plate is not the right direction. What you need is to have more capacity not reducing the capacity. If the existing pump exceeds the standard requirement, then do nothing!.

[kkala]

1. Sincere thanks to breizh for valuable information retrieved. Having looked at NFPA 20 (1999), following is understood.
α) Requirement that “pumps shall furnish not less than 150% of rated capacity at not less than 65% of total rated head” is written for centrifugal pumps (para 3-2) and again for vertical centrifugal pumps (para 4-1-2). Explanations are in Fig A3.2 of Appendix A (p. 20-49).
β) Specific instructions how to specify pump capacity and head (e.g. defining some safety margins, etc) have not been found, apart from the statement that “fire pumps shall have to be rated at net pressure of 40 psi (2.7 bar) or more” (para 2.3). This statement is not significant in most cases, since fire hydrants need a feed pressure of 8 Barg (NFPA reqirement?) to eject water at some distance through the hose, so discharge pressure has to be higher than 8 Barg (*).
2. I had erroneously supposed that above (α) is a supplementary requirement, while there was some guidance per (β) for pump rated data. But it seems that (α) is the only operational requirement (in addition to shutoff head< 140% x head, or equal).
Rated capacity & head apparently result from a fire fighting study, approved by authorities having jurisdiction. The pump curve has to pass from rated point and also fulfill (a).
3. Assuming that NFPA is the applicable code, authorities may not have the possibility to neglect (α). Thus insurance may not accept the pumps (as breizh warns) or ask for an excessive fee for equipment protected by them in firecase.
4. I believe actual rated data of the pump should be confirmed by test on field, if this has not been already. In case of inadequency, increase of RPM or impeller size (see S.AHMAD post) with its consequences can be the next step to look into.

Note (*): I have written an operating manual of a fire pump station (complying with NFPA recommendations), which fullfiled "minimum 8 Barg operating pressure at any point of fire water network"; no mention was made for (α), which was most probably covered by pump supplier. The lesson: be aware of the main relevant points of the code in reference from written sources, not from "gossip".

Edited by kkala, 23 August 2011 - 07:06 AM.

[lakshmi2011]

Thank you All for valuable opinions.

I am still wondering if we can do anything with Rated (Design)capacity as it is decided and NFPA 20 criteria is satisfied by the supplier ( as indicated by Kkala as well) .So if increase the demand required , do we still consider the 150% based on Rated capacity or new flow required, which means we are changing the rated values, which we are not supposed to !right ?

Further , NFPA 25 prescribes field tests for which the acceptance criteria for the FW pumps are :

1)The test is no less than 95percent of the pressure at rated flow and rated speed of the initial unadjustedfield acceptance test curve, provided that the original acceptance test curvematches the original certified pump curve by using theoretical factors.

(2) The fire pump is no less than 95 percent of the performance characteristicsas indicated on the pump nameplate.

so if the pumps are capable to meet the above criteria as well as meet required flow rate with adequate pressure , do we still need to go to meet 150% requirement with new flow ?

Regards,

lakshmi

and

1. Sincere thanks to breizh for valuable information retrieved. Having looked at NFPA 20 (1999), following is understood.
α) Requirement that “pumps shall furnish not less than 150% of rated capacity at not less than 65% of total rated head” is written for centrifugal pumps (para 3-2) and again for vertical centrifugal pumps (para 4-1-2). Explanations are in Fig A3.2 of Appendix A (p. 20-49).
β) Specific instructions how to specify pump capacity and head (e.g. defining some safety margins, etc) have not been found, apart from the statement that “fire pumps shall have to be rated at net pressure of 40 psi (2.7 bar) or more” (para 2.3). This statement is not significant in most cases, since fire hydrants need a feed pressure of 8 Barg (NFPA reqirement?) to eject water at some distance through the hose, so discharge pressure has to be higher than 8 Barg (*).
2. I had erroneously supposed that above (α) is a supplementary requirement, while there was some guidance per (β) for pump rated data. But it seems that (α) is the only operational requirement (in addition to shutoff head< 140% x head, or equal).
Rated capacity & head apparently result from a fire fighting study, approved by authorities having jurisdiction. The pump curve has to pass from rated point and also fulfill (a).
3. Assuming that NFPA is the applicable code, authorities may not have the possibility to neglect (α). Thus insurance may not accept the pumps (as breizh warns) or ask for an excessive fee for equipment protected by them in firecase.
4. I believe actual rated data of the pump should be confirmed by test on field, if this has not been already. In case of inadequency, increase of RPM or impeller size (see S.AHMAD post) with its consequences can be the next step to look into.

Note (*): I have written an operating manual of a fire pump station (complying with NFPA recommendations), which fullfiled "minimum 8 Barg operating pressure at any point of fire water network"; no mention was made for (α), which was most probably covered by pump supplier. The lesson: be aware of the main relevant points of the code in reference from written sources, not from "gossip".

[kkala]

I think rated data (capacity and head) of fire water pumps (FWPs) has changed to what you have specified after the installation of additional equipment (i.e. Q=1495 m3/h & H=11 bar). This has to be the result of a relevant fire fighting study, updating the old case; and this should be considered as rated data for the updated case. You have found out conditions for FWPs (either new or existing) to be suitable for the new service (per NFPA):
1. Condition from NFPA 20 : 1.5Q at 0.65H, which seems not to be fullfilled by existing pumps, as they are now.
2. Rated Q & H should be at least 95% of Q & H written on pump nameplate. Nameplate of existing pumps may report Q=1218 m3/h & H=12 bar. So tests on existing pump can be for H=11.4 bar at least (not 11 bar). On the other hand pump may operate far from its best efficiency point.
However communication with pump supplier may result in a new nameplate and some tests to support it.
3. NFPA 25, 1, seems to report how to "correct" test results, falling not far from the design point. Tests should be performed for a head of at least 95%H, then see whether the operating point is on the performance curve. It is useful in the pump tests.
It is noted that if you modify the pumps (e.g. by increasing impeller or RPM), these are apparently considered as new pumps. This is probably the next step to look into.
Above represent a non expert opinion, per my understanding.

Note on 28 Aug 11: underlined word changed from "operating" to "design" for clarity.

Edited by kkala, 28 August 2011 - 01:54 AM.

[ASSEM]

Hi
can anyone send me an excel sheet for fire pump and fire pump head calculations on my e-mail?.
i also need the same for chille water syystems.

assem.h@emecqatar.com

thanks for all.

[IUR]

Can any body guide me what pressure should be kept at farthest hydrant according to NFPA ( Refrence ?) . Any thoughts Please 

[breizh]

Consider this standard where you may find your answer .

 

Hope this help.

 

Breizh

 

[IUR]

Thanks Sir (Breizh)