NATURAL GAS PIPE SIZING (MEDIUM PRESSURE)

[NYOH]

Hello,

I have a question on sizing medium pressure gas pipe. Here is the information:

1. The total gas load is approximately 17,000,000 BTU which equals to 17,000 CFH.
2. Equipments minimum working pressure is 5 inch.H20 and maximum working pressure is 56 inch.H20.
2. Upon submitting this load to the gas utility company, they have told us that they will be providing a new 21/2 inch line with a regulator that will provide 2.5 PSI of gas pressure.
3. The property is a warehouse with a height of approximately 21 feet.
4. The total linear feet of pipe run from the point where the new 21/2" gas line will enter the warehouse to where the equipment is installed is approximately 150 feet.

I know that new schedule-40 pipe has to be welded. Looking at various "gas pipe design" tables, I can see that 21/2 inch line does not carry enough volume when 2.5 PSI is being provided by gas utility company. Do I have to up size the 21/2 inch line provided by the gas utility company after it enters the property to maybe 4 inch line so it can carry enough volume?

Thank you for your help!

 

[Drazen]

i would say that 2.5 psi is the value downstream the regulator, upstream pressure has for sure be much higher, i doubt gas company would make such mistake.

and yes, you need much larger pipe downstream the regulator.

that confuses many who are not familiar with gas to see pipe increase downstream, but that's common for any pressure stage change provided by regulator.

 

[PEDARRIN2]

Although 1 cfh = 1,000 btuh is a general rule of thumb, you might want to make sure of this. Natural gas from different locations can have different btu contents per unit volume.

For compressible flows, generally, the static lift is not considered unless the difference in elevation is substantial. One thing you might also want to consider is what the atmospheric pressure at your location is. This can have an impact on the calculations if you are at a significantly different elevation than standard sea level.

If the utility is providing 2.5 psig downstream of the service regulator, then you have a problem since the maximum pressure for your equipment is 56 in w.c. which is 2 psig. You will either have to get the utility company to provide a lower pressure or you will have to put a regulator at the equipment to reduce the pressure to a lesser amount.

 

[LittleInch]

It depends if your system needs 56 inches at the full flow or not.

Having said that 2 1/2 inches looks like it is really pushing it even with 50 inches water pressure drop to make 17,000 CFH.

Use 4" pipe but as pedarrin says, you need to check what the unit can actually withstand in terms of pressure and you might need a regulator yourself, even if it is only required at start-up / no flow conditions. If you go down that route then you might be able to ask for say 5 psig and run a smaller pipe to your second regulator upstream your equipment.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BronYrAur]

I only have experience with the gas utilities in Northern Illinois, but here we only have a choice of low pressure (typ less than 10"WC) or 2 psi. We can get 5 psi (or more) only if the process requires it. Having said that, 2.5 psi seems strange to me. I would request that they give you 2 psi, so that you won't need a pressure regulator at your equipment. Based on the info you provided, your equipment can handle up to 2 psi inlet pressure (56" WC).

4" pipe is the correct size for 17,000 CFH at 2 psi and 200 total equivalent length. I assume you don't have excessive fittings in your 150' pipe run, so your total equivalent length is probably around 200'. I would definitely recommend welded sch 40 black pipe.

 

[georgeverghese]

With 150ft of straight 2.5inch pipe and 100ft more for fittings losses, we get approx 55in WC at 21ft above ground level at full flow of 17e3 scfh.

If you work on a minimum 20in WC constant supply press to this equipment ( which should give you some room for low pressure alarms etc), then you've got approx 30-35in WC delta p for your onskid pressure regulator at 21ft elevation to flow 17e3 scfh to this equipment - fill up a PCV datasheet and take these to a PCV vendor and see what they say.

Be careful with turndown on flow - a bog standard regulator does not behave well with turndown. You havent said anything about min flow here.

So you've got to calculate pressure drop for 2 cases for this PCV - one at max flow and the other at min turndown flow, and put down these 2 separate cases on the PCV datasheet.

If your PCV fails wide open during turndown flow, pressure to your equipment will well exceed 56in WC - you may need a high pressure protection for this case if this is a problem ?

Is this equipment some king of fired heater ? How does it manage combustion controls ie. air - fuel ratio, or is this an induced draft heater - do you need another flow element in this line upstream of the PCV - more dp for this ? Do you need a guard strainer for the burner - more dp again - line pressure drop alone may not be all there is to this.