Engineering Units for Flow

Greg Dake

Member
Join Date
Jun 2005
Location
Milwaukee, Wi
Posts
550
Greetings,

On one of our process systems we have a sterile air skid which provides sterile air for a bottle sterilization process. The sterile air produced by this skid is later mixed with H202 for the sterilization process.

We are adding a couple of flowmeters to this skid. We need to measure air flow which is simple. The hard part is we'd like to be able to measure steam flow as well. The pipe will either have sterile air OR steam flowing through it. I know how to setup a flowmeter to measure only air flow or only steam. The problem is that I don't know of an engineering unit that can quantify both airflow and steam. The airflow and steam are potentially going to be a critical control point.

The easy solution is to add two flowmeters, one for the air flow and one for mass flow for the steam. We want to avoid another pressure drop a second flowmeter would add.

Another solution would be to infer how much steam was flowing based on pressure and the pipe diameter. The temperature sensor could ensure it was actually steam. The problem is that this would not be very accurate.

It really comes down to : Is there an engineering unit that can quantify both airflow and amount of steam past a given point with one flowmeter?

We're using ABB Swirl flowmeters FS4000/ST4's.

TIA,

Greg
 
I never used a meter with two different medias to be measured, but I would think a mass flow meter might do the job. I would contact E&H, Micromotion etc.... and see what they say. If you connect the meter via some type of digital bus, you could easily change the meter parameters on the fly.
 
Ken,

Thanks for the response. The problem is that, can you quantify air based on mass? How? Sure you can measure steam via mass flow, but air? I know what 100lbs/hr. of steam is, but what is 100lbs/hr of air?

Your other suggestion is something I had thought of because these flowmeters are HART capable. However since these are CCP's we'd rather not be changing parameters on the fly. An engineering unit that would satify quantity of air and steam would be best. The only thing we can think of is velocity, but again we'd be infering what the quantity is based on the size of pipe.

Greg
 
Sage Metering ( http://www.sagemetering.com/ ) has a thermal mass flow meter that has multiple channel capability. You can enter different properties and ranges in each channel. I believe you can either switch channels from the keypad or an external contact. They have 4-20 ma outputs for both flow and temperature on the same unit. Contact Bob Steinberg at Sage and tell him I sent you.

These flow meters inherrently measure mass flow rate, and you can set the outputs to pounds per hour for both steam and air.
 
Tom,

Thanks. I have never worked with Sage, we'll check them out. The thing I don't understant is, what is LBS/HR of air? I understand with steam, but not air. Wouldn't you have two different EU's for steam and air.....CFM for air and LBS/HR for steam? I don't know how CFM correlates to LBS/HR. Am I missing something?

Thanks again for the reference to Sage.

Greg
 
Any fluid flow can be measured as a volumetric flow rate or a mass flow rate.

This is easy with liquids. Water in gpm (gallons per minute) is a voumetric flow rate. Since one gallon of water = 8.33 pounds, you can get the mass flow rate by multiplying by the density. 100 gpm = 833 lb/min. 833 lb/min is the mass flow rate. If I was pumping 100 gpm of oil with a specific gravity of 0.80 the mass flow rate equivalent to 100 gpm would be 666 lb/min of oil.

You can use any length and time units for volumeteric (acre-feet per fortnight) or mass (long tons per nano-second).

Gases are compressible fluds, so it gets more complicated by the variation in density. The same principle applies, though.

You can measure steam flow rate in cubic feet per minute (CFM). If you know the temperature and pressure of the steam you can get the density, and then you can get the mass flow rate in lbs/hr or kg/day or whatever.

The same is true of air. You can mesure the velocity or volumetric flow rate in ACFM (actual CFM) or cubic yards per month or whatever. Knowing the temperature and pressure you can calculate the density, and then get the mass flow rate in lb/hr or tons/week or whatever.

Air is often measured in SCFM (Standard Cubic Feet per Minute). This looks like a volumetric flow rate, but it is actually a mass flow rate. That's because the S means the volumetric flow rate is corrected back to Standard Conditions of 68°F 14.7 psia and 36% Relative Humidity. That is a density of 0.075 lbs/cubic feet.

The thermal mass flow meter works on heat transfer, so it inherrently measures mass flow rate (the more molecules hitting the probe, the faster the heat transfer.) You can use the mass flow directly, or convert it to volumetric flow based on density calculated from temperature and pressure.
 
Great info

Tom,

As always, thanks for the good information. I (we), definately learned something. I have a question concerning your 2nd to the last paragraph in your last response. Where you talk about SCFM. You talk about standard conditions, 14.7psi and 68 deg F......what about density altitude? How would that be taken into affect? Would density altitude matter? (Just trying to further understand here, not questioning your last post)...

TIA,

Greg
 
ther is a useful tool that may help with some of the many calculations needed inour trade at www.pwr-tools.com its a handy engineering calculator some of the info contained within it applies to steam and air flow calculations
 
Greg Dake said:
what about density altitude?
At higher altitudes the pressure would drop so it would take more volume at a higher altitude to make a SCFM of air. The density is figured into the pressure. PV=nRT.

It is all about the relationship of pressure, temperature and volume of a given number of atoms of gas n in moles per degree K. R is a constant that relates the amount of energy per mole of gas. What you are really tryihng to do is measure the number of gas atoms. The pressure, volume and temperature are all you need.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
 
I'm also recommend talking with Micromotion as well, we have two of their "Elite" series metering systems in a new machine (purchased from an outside vendor), initially we had problems, they sent a field rep out to re-configure the transmitters to get us up and running, customer service is top notch.

In our setup we are measuring the flow rate of two very different liquid chemicals. The main chemical is used to produce our product, the second chemical is used to clean the system. Two very different chemical properties, but the meter handles them both with ease. But from what I've been told, these are not very cheap.
 
Greg Dake said:
Where you talk about SCFM. You talk about standard conditions, 14.7psi and 68 deg F......what about density altitude? How would that be taken into affect? Would density altitude matter?

The altitude effect is ceratinly a necessary part of the calculations, but it is inherrently compensated for in the calculation of density based on temperature and pressure. The formula from the thread referenced above is:

SCFM = ACFM x 2.69388 x psia / ((F + 460) x ).075)

The pressure used is pounds per square inch ABSOLUTE. That is guage pressure plus barometric pressure. Since the barometric pressure drops as altitude increases (about 1 psi per 2,000 feet) the altitude compensation is built in. Similarly, adding 460 to the temperature gives degrees Rankine, which is an absolute temperature scale. The 2.69388 value in the above conversion = 0.075 x (68+460) / 14.7 Note that this formula is only good for air based on these standard conditions.
 

Similar Topics

Just curious - I see there are channel configurations for the 5069-IF modules that let me type in the high & low Engineering units as well as the...
Replies
3
Views
951
Hi, Our PLC measures WATER FLOW RATE as an analog input. The PLC reads it as an integer value (D34) in the range 0-4000 as follows: Range...
Replies
12
Views
2,523
Context: Mitsubishi Q series Hi, today I configured an analogue input into the PLC. Then I injected 4-20mA at the terminal block. The digital...
Replies
47
Views
11,421
I need to scale a raw value into Engineering units. i don't think there is a standard scaling block in PLC 5 instruction set (or i didn't see...
Replies
19
Views
7,110
Hi I am newbie, I want to know the formula for converting raw counts to Engineering .Lets assume that the a temperature range of 0-100...
Replies
19
Views
5,709
Back
Top Bottom