Orifice plate flow measurement.

We are undertaking a job consisting of replacing a gas valve motor and air damper on a burner with a control valve for the gas and variable speed drive for the air flow control. Flow feed back is to be accomplished with orifice plates and d.p. transmitters. There is no question of using anything other than orifice plates, this is a customer spec. My questions are:
1.To calculate the flow in the plc (Clogix)is it simply a matter of finding the square root of the differential pressure across the plate. I have seen so many interpretations of different formula, I really need some help.
2.If any body has any experience of a similar scheme I would be really grateful for advice on pitfalls etc.
Best Regards

Hi Pauly,

Not sure if I can help it may be over my head, but I would be interested in more detail on the application, I know you need to use the customers specs, but I have never heard of using orifice plates in this type of application, is it for a boiler of sort or incinerator?

The orifice plates I have always used have been constant not a variable.

Are the plates to limit the flow of gas? I have always used a positioner (Honeywell) with limit parameters inside the controller

Also on the air flow monitor I use a pitot tube

Here is a thread with the formula to calculate SCFM that I used it worked very good, there is one calculation that can be a constant, if you need the code itself I can post it..

http://www.plctalk.net/qanda/showthread.php?t=26650

Mark

This sizing of an orifice plate is a straightforward calculation. You need to input your dp signals from your transmitters to the PLC, take the square roots and multiply the results by the flow factors obtained from the orifice plate calculations which gives you the flow rates.
The first question is do you require to measure the mass flow rates, volumetric flow rates at operating conditions, or else volumetric flow rates at standard conditions, and are you using imperial units or metric units? The answers to this question will determine what process conditions will be required to do sizing calculations for the orifice plates.
Data required to do the orifice sizing calculation is as follows:
Pipe size, schedule, and material.
Fluid state: Liquid or gas.
Required flow rate at 100% ouitput of dp transmitters.
Operating pressure upstream of orifice plate.
Operating temperature upstream of orifice plate.
Operating density of gas / liquid.
Density at reference conditions (P and T).
Viscosity of fluid.
Ratio of Specific Heats if fluid is a gas.
The calculation could then be done to find the orifice size, determine a suitable differential pressure (dp) and the applicable flowfactor.

Flow calculations

Sorry guys I think I may not have explained properly. I have been tasked to do the PLC software to control the temperature inside a vessel which is heated via a gas burner. The orifice plates are being sized/supplied by others. What I was asking was for info on the calculation I need to do to convert the analogue input from the d.p. transmitters to a flow.
P.S. there will be a seperate flame supervision unit, I will not be doing that in the PLC.
Any help with flow calculations using orifice plates will be gratefully received.

I really should read the posts!

Sorry Calistodwt I should have read your reply before diving in. Thanks for the info, can I assume then that I will receive info from the orifice plate supppliers which will contain a flow factor?
I multiply this by the square root of the pressure drop from the d.p transmitters?
i.e. Flow= flow factor*sq root(p1-p2)

Not quite as you indicate! The orifice calculation will determine the maximum differential pressure(dp) at the maximum required flowrate. For example the calculation may give dp range as 0 to 100 inches Water gauge for a required flowrange of 0 to 300 cubic feet per hour. Now you need to use the dp transmitter input as a per unit value i.e. 0 to 1 where 0 = 0%, 0.5 = 50%, and 1 = 100%. So at maximum input you take sqrt(1) * 300 = 300 cu.ft/hr. As a check at 25% dp = 0.25 per unit. So sqrt(0.25) * 300 = 150 cu.ft/hr which is correct (for a dp 0f 25% you always have 50% of maximum flowrate because of the square root relationship).

To put it simply the orifice plate info should have a max flow rate and a max DP, usually in inches water (or for the UK, sometimes Millibar or Kpa)

This is what the DP transmitter will be calibrated to.
Check and see if the transmitter is doing the square root, if not that will have to be done in the PLC.
Personally I prefer its done in the transmitter, less work for you and better signal accuracy.

Then all you have to do is scale the PLC 4-20 ma input to the same range as on the orifice plate calc sheet.

Job done.

I agree with SLC Integrator. Almost all DP transmitters will extract the square root at no extra cost. You simply get the orifice plate supplier to tell you the differential pressure at the max flow rate you require, then set the transmitter to give you 20 mA at this value with the square root in the transmitter.

The transmitter and orifice data sheets should give you this info. For convenience I usually get the transmitter from the orifice plate supplier pre-calibrated in this fashion.

I've done it the other way as well, and it is fairly straight forward but somewhat time consuming to do the calculations.

cheers guys

Thanks guys for taking the time to give the benefit of your experience. I need to get some quotes for orifice plates and transmitters from the same supplier.
Thanks again

Pauly

I've been involved in numerous burner applications with electronic fuel/air flow ratio control using O plates and DP transmitters and can say that there are plenty of pitfalls.

These systems are not very forgiving if not done properly. Off the top of my head here are a few things to watch out for:

1. Correctly sized, manufactured and installed orifice plates.
2. Pressure tappings correctly installed.
3. Good quality, accurate DP transmitters (preferably with square root extraction built in as said by others) properly calibrated.
4. Don't expect turndown to be fantastic, remember 1/10th max. flow = 1/100th max DP
5. Ensure that there are sufficient straight pipe runs up and down stream of the orifice plates.
6. Correctly installed impulse lines from the pressure tappings to the DP transmitters.
7. Good gas supply pressure regulation.
8. Correctly sized gas control valve and actuator with good quality positioner (pneumatic are generally quicker and more accurate).
9. Consider speed of response and lead/lag (gas or air lead?) which will cause the burner to go rich or lean during changes to burner throughput.
10. Tuning has to be good and setting up the gas flow control loop may be difficult if large variations in air/gas ratio cannot be tolerated.
11. Consider oxygen trim to correct inaccuracies and give better fuel efficiency - especially if this is a large burner.
12. Consider failure modes, e.g. gas DP transmitter fails and gas valve winds fully open.

An electronic flow ratio control system, properly installed and commissioned will work well but I've yet to see one myself which has performed significantly better than a conventional ratio regulator that the likes of Kromschroder make.

Thanks for the heads up!

Thanks for the heads up Burnerman, the trouble is that my boss sells these ideas and poor sods like have to make them work!
I would be grateful for some part numbers for ratio controllers that you have used successfully before this goes too far!!

Any single loop controller is generally capable of ratio control.

I have a couple of ABB 355 Commander controllers sat right here on my desk that have the capability.
Yokogawa make a good single loop controller (US1000), as do Honeywell (UDC Series).

The most complete burner air/fuel ratio control scheme is a cross-limiting system where high and lo select blocks will prevent either of the two loops from winding up and giving a potentially dangerous air/fuel mixture.

Unfortunately its really hard to build a scheme like that outside of a high-end PLC or DCS.

OK, sometimes I'm not a genius, but I try...

kind of off topic but...

How in the heck do you control a burner with a orifice plate? I still don't get it, I have search around a bit but have not found anything that clicks with any of my applications, I have globe valves and butterfly on my burners.

I have used blinds with orifices (not a blind anymore) to restrict the flow oil and air, but how can it be a variable? are they like shutters? or am really far off on this?

Anyone got a dumb down link or explanation?

thanks GiT

GIT,


You're just a little confused...
The orifice plate with dP transmitter is used to measure gas flow. It is not the final control element. The original post stated he was using a control valve to control gas flow.

We are undertaking a job consisting of replacing a gas valve motor and air damper on a burner with a control valve for the gas and variable speed drive for the air flow control. Flow feed back is to be accomplished with orifice plates and d.p. transmitters.

Click to expand...

The most complete burner air/fuel ratio control scheme is a cross-limiting system where high and lo select blocks will prevent either of the two loops from winding up and giving a potentially dangerous air/fuel mixture.

Unfortunately its really hard to build a scheme like that outside of a high-end PLC or DCS.

Click to expand...

The Siemens (formerly Moore) 353 single loop process controller is routinely used for cross-limiting or O2 trim in boiler control. But it is a high end controller at $1,700 USD