I answered a question via E-mail again, and thought that I might post it here.
“I couldn’t use a condensate meter as a utility billing meter for steam. That is my whole argument.”
Just a quick note:
I saw the email trail between Steve and yourself, and thought that I would add my thoughts. Our meters are only used for utility billing. Not just sometimes, but all the time. We don’t have any installations (that we know of) that are not used for utility billing.
Many times, we are installed on a system where a vortex shedder is already reading steam flow. We’ll read the same flow, after the users have condensed. In these systems where there is both a vortex shedder and a gravity meter, the vortex will be used for instantaneous flow rate, and the gravity will be used for billing.
The reason is that the best vortex shedders only have a 24:1 turndown when matched exactly for pipe size vs flow rate. But that’s a rare occasion. Vortex meters are always sized for minimum pressure drop, and thus are commonly oversized. They don’t make pipe sizes for specific flow ranges, so we’re stuck with standard piping sizes. After sizing, most applications average around 12:1.
I just sized a steam vortex meter for a project at a Fortune 100 company. Parameters were 6″ pipe, 240 PSIG saturated steam, max flow of 14,000 PPH. Now, we commonly size steam for a flowing velocity of 130 feet per second. This keeps pressure drop and noise levels at acceptable levels. At that velocity, a 6″ pipe will flow 47,000 lbs/hr, but minimum meter read on a 6″ is 1,950 lbs/hr. This is an industrial installation, therefore not subjected to winter-summer flow rates like a building will experience. But even at that, their low flow rates ran lower than 1,000 lbs/hr, a reasonable 14:1 turndown ratio in this instance.
This is a pretty normal situation. You are obviously pretty versed in this (I googled you) so you already know that all pipe is oversized to minimize transport pressure drop. 6″ pipe is a pretty reasonable size for this flow rate. But a 6″ meter is a clear mistake.
In this case, we recommended installing a meter run of either 4″ or 3″ pipe with a corresponding meter size. The 4″ meter will pass 20,500 lbs/hr at our 130 FPS number, and will read down to a minimum of 860 lbs/hr for a turndown of 16:1 at their high flow.
The 3″ will flow 11,500 lbs/hr at 130 FPS, and would flow well beyond that if pressure drop and noise are not a question. Their average flow rates were about 8,000 lbs/hr with only three spikes of 14,000 lbs/hr in a year’s time. This meter would read all the way down to a low flow of 484 lbs/hr. We’ve reached the theoretical 24:1 turndown, but only after sacrificing our 130 FPS standard. And there’s that bottom again.
In contrast, a gravity style flow meter will read the maximum flow that it’s designed for, and then sacrifice any flow over that number. But gravity meters have no low flow cutoff. It could only be a drip a minute, and a gravity flow meter will still read it. This makes meter sizing quick. Just size for high flow, no more deciding where to sacrifice.
And that’s the difference. All velocity based flowmeters have a low flow rate that they no longer can read. We are based on weight, and therefore have no low flow cutoff. Our losses are only at high flow. Thus, we can oversize the meter to catch all the flow without ever worrying about missing low flow numbers.
That’s why we are used for billing. Just like the gas meters that will read a pilot light, nothing goes uncounted. When flow rates drop to 1 lb per hour, our 16K lb/hr meter is as accurate as our 250 lb/hr meter.
Gravity meters were first patented in 1859. A number of improvements were made until the final form was patented by Redfield and Bordley of Cadillac in 1912. Over a hundred years later, we are still selling gravity style meters because of their limitless turndown. Energy is expensive. And if you want to read it all, gravity is the only meter out there.
Well, maybe that wasn’t such a quick note, but hopefully I wasn’t too long winded.