Hybrid MPG

The other day one of our estimators ran a gas system design past me that came across his desk from another engineering firm. It was an interesting approach that left me wondering if it was more or less expensive than a conventional design. Here in SoCal we divide our gas systems into two basic categories, low-pressure (conventional) systems and medium-pressure (5 psi) systems. The code has natural gas sizing charts taken from NFPA 54 in Chapter 12 for sizing each of these systems. The gas company will only allow us to utilize medium pressure (MPG) when the load for the building is sizable, making the low-pressure piping impractically large.

 
The two sizing tables used most often in Chapter 12 of the UPC (from NFPA 54) are Table 1216.2(1) for low-pressure (under 2 psi with a maximum 0.5-foot w.c. pressure drop), and Table 1216.2(6) for medium-pressure (5 psi with a maximum 3.5 psi drop). However, there are four tables in between that are not often used for differing pressure conditions.My design estimator showed me this.

The four tables in between the low pressure and medium pressure tables are for 2 psi gas with a 3-foot w.c. drop; 2 psi with a 6-foot w.c. drop; 2 psi with a 1 psi drop; and 3 psi with a 2 psi drop. Hmmm … How best could one utilize these options?

Except for unusual situations, such as a generator that requires 5 psi gas, the distinction between a low- and medium-pressure system (other than pipe sizing) is the need in medium-pressure systems for pressure-regulating valves before making connections to equipment requiring low pressure gas. This is very common with big boilers and water heaters that have large gas demand — to have MPG with a regulator ahead of the equipment. Note that with today’s high efficiency, low NOx equipment, it is wise to provide a regulator dedicated to each piece of such equipment to prevent flame-out conditions on start-up. 

So, in between these two gas system options lie several other alternatives that are not often utilized. Do they make sense? I’m not sure yet. Let’s call them hybrid MPG systems for the sake of this article. These would be systems that utilize less than 5 psi supply but greater than low pressure. In the case of the design my estimator showed me, the engineer had selected 2 psi distribution with a ventless regulator behind each appliance. 

There are pros and cons behind this idea. On the pro side, there are benefits to utilizing the other gas sizing tables in the code and associated pipe sizing economy. If one were to use Table 1216.2(4) for 2 psi gas with a 1 psi drop, a 1-inch pipe 100 inches long could supply 1,710 CFH. In contrast, a low pressure pipe of the same size and length could deliver only 195 CFH – almost one tenth the capacity. The same capacity on the low pressure side would require a 2.5-foot pipe. This is quite a difference.

On the con side, the hybrid system requires a ventless regulator at each appliance, and these carry both material and labor cost, and I doubt a ventless regulator could fit tucked behind your average range. Further, our SoCal gas company requires that regulators be accessible for service at 4" above the floor. This means each appliance would require a serviceable area for each regulator — not very practical.   

Another con to the hybrid system concept is that medium- (greater than low-) pressure systems have to be tested at 60 psi, and it is very hard to pass a pressure test at 60 psi of air with threaded pipe. As a result, we normally run medium-pressure systems with welded connections, so the hybrid would require welded joints at every fitting. This adds cost and labor compared with the threaded fittings of a low-pressure system.

Another con to the hybrid system is the gas company’s reluctance to allow anything other than low-pressure gas to be used for other than high-fire equipment. There is a reluctance to allow it to be buried in the walls of residences. In fact, I did a project in Dallas many years ago where the local gas company required the medium pressure gas pipe to be installed outside the building envelope and hidden in an architectural enclosure, but I have not run across this requirement outside Dallas. 

Using another city as a contrasting example, New York City doesn’t have medium-pressure gas. In fact, when I left there 16 years ago the gas company wouldn’t guarantee anything over 4.5-inch w.c. of pressure. Many appliance manufacturers won’t even provide a warranty for their equipment for such low gas pressure, but somehow things always worked out in the end, since appliance manufacturers surely didn’t want to lose the NYC market. Funny how that works.

Speaking of NYC, we often discussed but never really quantified the fact that for tall buildings, natural gas increases in pressure as it rises in elevation, due to the fact that it is only 60 percent as dense as air. This rise in pressure acts to counteract the friction loss in vertical gas risers. However, I have never taken the time to calculate the extent to which that could benefit the sizing of a vertical gas distribution system, primarily because I know that our local officials here in SoCal would never buy into it. If it’s not in the code, they want no part of it.
So as an engineer it is good to know that we have options, such as a hybrid gas distribution system, or vertical gas design where the pressure gain helps offset friction. It might be hard to justify a design that is not in the code verbatim, and it might be questionable whether the addition of local appliance regulators outweighs the benefits of reduced gas pipe sizing. But these are things worth considering as you go through the engineering process. 

Complicating things further, here in Los Angeles there is a new high rise ordinance that requires new skyscrapers to meet a fire department blast ordinance. I imagine other major cities have similar requirements. The ordinance affects gas piping design, as it doesn’t allow gas piping to cross the protective shear wall enclosure of the elevator core. The full details and ramifications of this ordinance are still unclear to me, but I will figure it out soon enough.

As a final note, if you do decide to design a hybrid gas system, be sure to select your regulators accordingly. If you select a system that is 2 psi with a 1 psi drop, the regulators have to be sized for a 1 psi inlet pressure (the difference between 2 psi and 1 psi). Good luck. 

Timothy Allinson is vice president of Engineering at Murray Co., Mechanical Contractors, in Long Beach, California. He holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a LEED accredited professional. Allinson is a past-president of ASPE, both the New York and Orange County chapters. He can be reached at lagunatictim@gmail.com.

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