Flow Meter Selection & Approximating Flows
Feature: Considerations in Flow Meter Selection
CONSIDERATIONS IN FLOW METER SELECTION
Selecting a flowmeter can be challenging given dozens of meter types and hundreds of manufacturers insisting their meter is "best." In truth, there is no "best" meter (including MEMFlo) given the infinite combinations of fluids, pressures, temperatures, and application requirements possible. Your application requirements need to be the focus of meter selection. The following information should always be known to select the proper flow meter:
- Fluid name;
- The fluid specific gravity (or density);
- Flow range to be metered (even needed for flow totalizing applications);
- The operating pressure and temperature of your application; if these will change, the minimum, normal, and maximum values should be communicated to the flow meter supplier;
- Material preferences, particularly in the case of corrosive fluids;
- Pipe size and/or connection requirements; although, as discussed below, that may need further consideration;
- If electronic accessories, such as 4-20mA outputs, remote totalizers, alarms and the like, are required, what is the available input power;
- Unusual circumstances, such as dirty fluids, heat sources, environmental concerns that might include weather exposure, powerful magnetic fields, hazardous areas, excessive vibration, or spacial limitations are just some examples.
Let's talk about some of those "unusual circumstances" and how MEMFlo has handled some.
Piping
While it is a consideration, pipe size should not taken as an "absolute" factor. MEMFlo once had a customer insist on a 2" meter for a 50 GPM application that would have been ideal for one of our standard 1-1/2" Thru View models. Because they insisted, we provide a 2" Series T meter at considerably higher cost.
When visiting their plant some time later, we happened by the meter it had been installed on one inch pipe! It turns out they had another meter previously installed, and because that one was a 2" model, they thought they had to replace it with another. Obviously, they would have been better off with the less expensive 1-1/2" Thru View.
It's a good idea to keep the following in mind when it comes to piping:
- As a general rule, if you are using metal pipe, stick with metal-bodied flowmeters. If your piping is plastic, you'll probably want a meter body of plastic. Never install threaded metal pipe into a threaded plastic meter body (unless you want to buy replacements often)
- For most applications, moving up or down a pipe size won't hurt, especially if it saves you money or offers some other advantage. The exception would be a gravity feed or other low pressure system where pressure loss of additional fittings or reductions can cause problems.
- Where there's a will, there's a way. Flow meters come in a variety of piping arrangements, such as vertical in horizontal out, vertical in vertical out, horizontal in and out, and so on. Usually there's a location where any meter can be readily installed, and most pipe fitters are quite adept at handling this issue. For example, some people might tend to think MEMFlo meters don't work with horizontal pipe lines. As you can see by the photo on the right, that's not the case.
Dirty Fluids
The nature of the dirt should be considered large, abrasive particulates in the liquid can cause problems with many meters. In general with MEMFlo meters, the larger the meter and flow, the more forgiving the meter to "dirt."
As far as visibility, even our meters with transparent meter tubes are often satisfactory, as seen in the un-retouched photo at the right. (Click on the image for a larger view.) If the fluid is even darker or opaque, one of our meters with a magnetic indicator may be the best choice.
Heat
It isn't only the media temperature that can impact flow meter selection. Meters used near boilers or furnaces may be subjected to a great deal of heat from the surroundings. This heat may not be sufficient to compromise the meter mechanically, but it can cause major problems with electronics.
Locating the meter further from the heat source is a good idea if possible. Otherwise, MEMFlo may be able to offer a solution, such as the heat shield option show on the meter to the right.
Other concerns are readout options. These may be as simple as reading the rate of flow on an analog indicator or total on a mechanical register, or as complex as having sophisticated control options including electronic outputs, alarms, totalization, and/or dynamic compensation for changing fluid conditions.
MEMFlo will look at those factors in a future issue of this newsletter.
TECH AIDE: APPROXIMATING FLOWS
If you call MEMFlo for a flowmeter, we're bound to ask you, "What's your flow?" Now and then, the answer given is, "If I knew that, I wouldn't need a flowmeter!"
So what can you do when they need to select a flowmeter, but do not know the flow? An estimate may be practical, particularly since MEMFlo Flowmeters offer both wide rangeability and easy field changing of internals to alter capacity.
Liquid Service
There's no easy way to estimate liquid flows. If it's a small enough flow and the liquid isn't hazardous, a "bucket and stopwatch" approach will do the trick. Timing how long to fill a bucket, 55 gallon drum, or other container of known quantity or dimensions can be converted to a close flow rate estimate. But if that approach is not an option, you'll need help.
Your pump distributor or manufacturer may be able to help. By providing them with the discharge gage pressure under operation at normal and maximum flows, pump model and data numbers from the plate, horse power and RPM, and the liquid being pumped, they should be able to give a good approximation of flow.
If the liquid is fuel for a furnace, or feed water to a boiler, the Btuh ratings should provide some guidance as shown in the tables below.
Gas & Steam Service
Obviously, a bucket and stopwatch approach won't work for you here. However, there are some reasonably good rules of thumb for compressed air flows based on the size and type of compressor being used. With steam, the boiler ratings should, again, provide some guidance. The tables below provide further detail.
FLOW ESTIMATING DATA
|
Altitude, Feet Above Sea Level |
Single Stage Compressor |
Two Stage Compressor |
|||||
|
60 |
80 |
100 |
60 |
80 |
100 |
125 |
|
|
0 |
5.5 |
4.6 |
4.1 |
6.8 |
5.8 |
5.2 |
4.7 |
|
2000 |
5.4 |
4.5 |
4.0 |
6.7 |
5.7 |
5.1 |
4.5 |
|
4000 |
5.3 |
4.4 |
3.9 |
6.5 |
5.5 |
4.9 |
4.3 |
|
6000 |
5.2 |
4.3 |
3.7 |
6.4 |
5.4 |
4.7 |
4.0 |
|
Steam |
For 100,000 BTUH Input @ 75% Efficiency |
||
|
PSIG |
SAT. Temp. °F. |
SAT. STEAM, LBS/HR1 |
MAX. GPH FEED WATER2 |
|
40 |
287° |
66.0 |
7.9 |
|
60 |
307° |
65.6 |
7.9 |
|
80 |
324° |
65.3 |
7.9 |
|
100 |
338° |
65.1 |
7.8 |
|
200 |
387° |
64.6 |
7.8 |
|
300 |
422° |
64.4 |
7.7 |
|
1 Assumes 100% make up feed water @ 70°F. |
|||
|
FOR GAS: Read input in Btuh from nameplate. If only output is shown, divide by 0.75 (modern equipment) to 0.65 (old equipment) to obtain input estimate. |
FOR LIQUID: Again, read or develop input divide by 0.70 (new) or 0.60 (old) to obtain input from output values. |
|||
|
TYPE OF GAS |
SCFM/100,000 BTUH INPUT |
GRADE OF FUEL OIL |
GPH/100,000 BTUH INPUT |
|
|
Natural gas (1050 Btuh/Cu.Ft.) |
1.587 |
#1 (135,000 Btuh/Gal.) |
0.741 |
|
|
Propane (2520 Btuh/Cu.Ft.) |
0.661 |
#2 (139,400 Btuh/Gal.) |
0.717 |
|
|
Butane (3260 Btuh/Cu.Ft.) |
0.511 |
#4 (145,000 Btuh/Gal.) |
0.686 |
|
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