Natural Gas Pressure Regulator Turn Down Ratio

[PEDARRIN2]

This question is geared toward building natural gas regulators where the inlet presssure is likely to max at 10 psig and be regulatored down to 7 in w.c. Flows are going to be on the low end, likely maxing out at around 10,000 cfh.

I am familiar with natural gas equipment turn down ratios but have never seen the term be related to the regulators. With equipment, this is always a flow parameter, but doing a little research, I have found guidance that indicates it to be the ratio of the inlet/outlet pressure. Not sure if that is absolute pressures, but with 10 psig maximum and regulate down to about 7 in w.c. (0.25 psig). So that would be a 40:1 turn down ratio if using pressure.

But other guidance indicates it to be the flow ratio.

I have seen references that indicates the regulator should be sized for 10:1 turn down flow ratio. For a building, that might also be difficult since the regulator may be required to control pressure at flows of 120 cfh (water heater) all the way up to 10,000+ cfh (boilers, kitchen equipment, water heaters) which would exceed the 10:1 ratio.

A lot of the regulators I commonly specify do not even mention turn down ratio in their literature.

Any guidance would be appreciated.

 

[LittleInch]

Most regulators like this will or should be Tight shut Off (TSO) so that at zero flow there is no weeping or leakage. In theory many would probably claim 0-100% flow range.

The issue after that is how much volume you have downstream your regulator to act as a buffer to stop the regulator opening and closing / fluttering at very low flows, say <1% of max. But anything between 5 - 100% I would anticipate no issues.

You probably need to try and find the technical sales person who can tell you, but my guess is they will say there is no flow range other than 0-100%.

for larger kit the 10:1 is a good way of looking at it and then install a smaller one to deal with the smaller flows.

For mechanical regulators I've seen a smaller one set at a high pressure so that it flows 100% at higher flows, but as the flow reduces and the pressure backs up a bit the larger on cuts off, but the smaller one set 10% higher continues to operate and regulate more accurately at the lower flows.



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[davefitz]

The turndown may be based on Cv ratio, max to min, but sometimes is based on flow ratio if the upstream conditions are roughly maintained at a constant value.For large nat gas valves it is feasible to have a "characterized trim" that provides a very high theoretical turndown but these are not usually tight shutoff trims, such as the CCi self-drag valves.Little inch provides some good recommendations where a small valve is in parallel to a larger valve for a wide turndown.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[PEDARRIN2]

I have used a parallel arrangement for water pressure regulators inside a building but have never used (or seen) parallel regulators for natural gas. It might be we never see large enough flows to really require them, or we just did not think to apply them in that way.

Definitely food for thought.

 

[georgeverghese]

For control valves, turndown on Cv is typically 25:1. For self operated spring loaded regulators, it is much less, and I dont remember the turndown range for these. But you get better turndown with pilot operated regulators (and less droop) than with non pilot operated ones. If set point droop cannot be accepted at high flows(as will be in most cases), use pilot operated regulators. To enable better control stability with pilot operated regulators, use the 2 path pilot option - see Perry Chem Engg Handbook, page 8-81, Chap on process controls, subsection on regulators.
Ask the regulator vendor what turndown limits are for these. Turndown flows and limitations in setpoint droop should be specified in process instrumentation datasheets. You may need to enable overpressure protection on the downstream side to account for either of (a) the regulator failing wide open or (b) regulator lockup at very low to zero flow.