Request for help on accurate steam flow measurement.

[luciom]

Presently we are using a 4" Vortex flowmeter that is generating high error which the manufacturer says is
due to oversizing of meter. The flow varies between
600 to 3000 Kg/hr.. Is there a meter that can cover
this range accurately ?

 

[quark]

Luciom!

Recently I have excercised selection and installation of steam meters. If I recall correctly at 10Kg/Sq.Cm (g) pressure a 3" meter would satisfactorily operate in the range what you have specified.

You have to tell me the following things.
1. What is the steam pressure?
2. Whether it is saturated or not?
3. Have you gone for density compensation or not? if so what parameters are being compensated?
4. What is the free length you have provided?

As I had to browse through morethan 500 A4 size pages for this selection (total 23 meters) more questions can be followed

Or else you can use corrialis mass flow meter.

Regards,

 

[TD2K]

You are only talking about a 6:1 turndown for your flow rate, a vortex meter should easily cover this. However, vortex meters also have minimum and maximum flow rates so it sounds like the vendor is telling you this 4" meter is too large.

As quark said, you'll need to provide a lot more information for any specific recommendations. What is the make of your existing flow meter?

 

[nitinp]

Quark,
Meter we are using is Krohne Marshall Model VFM 5090 SSM.
Meter Size 4". Pressure 10 Kg/ Sq.Cm. Saturated steam.
Density compensation : Provided (Temparature).
Straight Lengths: Upstream 20D
Down Stream: 10D.
As per the manufacturer measuring range is: 659.72 to 9681.78 Kgs/Hr (@10 Kg/Sq.Cm).

 

[hiteshvakil]

Hi Luciom,
I am also a regular user of steam flow meter but we are using orifice type.
In this system Differencial pressure transmitter's range of 4 to 20miliamps. can be set for lowest and highest value of flow.
Please note that this is also depending on Impulse piping layout and selection of condensate ports.
You also use totalizer for average reading and covert from RMS value.

 

[cymro]

Luciom,
Agreed -turndown is not a problem here. Your 4" vortex meter is oversized, this almost certainly means that the necessary velocities for efficient vortex shedding are not being achieved. For a vortex meter, as said above, 3" is a better choice for this application, why not go back to the manufacturer and try and come to a deal?. Alternatively, if your geared up for 4", an alternative is a 4" variable orifice meter. This would more than cover your range with complete accuracy. Steam velocity at 10 kg/cm^2 would be around 20 m/s at 3 000 kg/h, and pressure drop across the meter would only be about 20 kPa (3 psi / 80" w.g.) at this flowrate - well within good practice for saturated steam, and probably within your specification. No problem with up/down pipe diameters either. I suggest though that you go to a Company of renown who is foremost 'expert' in steam system applications and who manufacturer such meters. I find that these sort of people know steam applications and can anticipate the problems you've experienced, rather than 'pure' meter manufacturers who tend only to know their product.

 

[cymro]

Luciom,
Further to hiteshvakil's reply, please be careful - a simple orifice plate may not give you the accuracy you require at your low flowrates. These meters only have a reliable turndown of about 4:1, this being due to the square root relationship between flowrate and pressure drop. This is not a problem at the high end, a small change in flow is represented by a relatively large change in pressure drop, but at low flows (with the same size hole), the same small change in flow is represented by a much smaller change in pressure drop. The maximum flow you need to measure is 3 000 kg/h, typically, the minimum is going to be about 750 kg/h. You are only measuring a pressure drop of about 1.7 kPa (7" w.g.) at 750 kg/h flow, so you can appreciate the difficulty of a DP transmitter to register a typical change of about 1/2" w.g. for a change of about 50 kg/h either side of this.

 

[TBP]

I formerly worked for a steam utility where we sold steam to various customers. If the condensate is being recovered, the most accurate method of measurement is condensate metering. Very simple technology (1920's), accurate to within 1% and an infinite turndown. Lincoln and Cadillac are two companies in the U.S. (Do NOT use an ordinary water meter.) They can read high if they're installed on (for example) a steam to liquid heat exchanger, and there's a leak that lets water into the steam side. The meter has no way to know where this liquid is coming from. It sees liquid, it turns.

For steam metering, we used vortex shedders with pressure compensation. Pressure comp is critical when accuracy is important. Meters are similar to control valves in that if the line is sized correctly, it's a pretty good rule of thumb that the meter will be one pipe size smaller.

Also note that, vortex shedders will quite happily read mechanical vibration as flow. Things like thermodynamic traps discharging close to the meter, nearby pumps, etc can affect readings. This really shows up during periods of low or no flow. If you're billing someone off this reading, it can lead to some interesting "discussions".

 

[quark]

Luciom!

Orifice plate meters (even the models of F&R with density compensation) can have a maximum turndown ratio of 8:1. The basic inaccuracy of DP meters is very low. It is rather better to say any compensation would reduce error of measurement rather than increasing the accuracy in case of DP meters. (GOd forbid if anyone takes a literal meaning of it)

In this range a DP meter is not a good option.