Use of orifice plates for balancing water spray/deluge systems

[Guidoo]

As per NFPA 15 (Water Spray Fixed Systems for Fire Protection), section 8.5.3.9 "Orifice Plates shall not be used for balancing the system".

In practice, I know that orifice plates are often used for this, or as an alternative, (custom made) small bore spool pieces are used. In case you really do not want to use these options, you have to use lots of different pipe sizes, including non-standard sizes such as 5 or 2 1/2 inch pipes. Piping engineers will not like this...
Also you highly rely on your hydraulic calculations that are in fact only an estimate (e.g. use of C factor in Hazen Williams calculations, in practice the C factor will be different from what you used in the calculations...)

NFPA 15 probably forbids using orifice plates because of possibility that these plates get plugged. Advantage is though that you can use standard pipe sizes only and that you can easily adjust the orifice size when your hydraulic calculations were not completely right.

What are your experiences and opinions on this issue?

 

[stookeyfpe]

You answered your own question -- orifice plates can be obstructed and they are prohibited by NFPA 15 and NFPA 13, which is the standard for automatic sprinkler systems.

I'm a little confused over the statement that you can adjust the orifice size when the calculations are incorrect. The goal should be to provide the required discharge density to protect the hazard in question. Hazen-Williams offers a fairly robust design factor to offset differences in pipe diameters so long as a conservative pipe roughness factor (C) is used.

And while piping engineers may not like using standard pipe diameters, the fabricators of the pipe will be happy, which should in turn, make erection a standard event for the pipe fitters. This will hopefully result in a properly designed system for hazard of concern.

 

[Guidoo]

stookeyfpe,

Thank you for your reply. I have a few remarks:

NFPA 15 and NFPA 13 can only prohibit things when the plant is built in a place where NFPA is a code, e.g. the USA or when the insurance company requires that NFPA is strictly followed. This is not always the case (e.g. when plant is outside USA and company is self-insured).

When we are worried about obstruction of the orifice plate, we should also be worried about obstruction of the spray nozzles (that are in effect orifices as well...). Therefore we do proper material selection and install strainers at the deluge valve. Also, NFPA does allow use of small bore spool pieces, that can be obstructed as well. System should be tested frequently to detect any problems such as blockages.

About my statement that orifices can be adjusted if calculations are not fully correct in practice, I meant that when pressure drops are too small (e.g. because conservative C factor was used in calculations), pressures at the spray nozzles can be too high (e.g. >4 barg). This results in formation of too small water droplets (mist) that are not effective in cooling the equipment. When you have used orifice plates you simply replace the orifice.

I wrote that piping engineers do not like non-standard pipe sizes (such as 5 inch).

I am interested in your view on this.