I have installed Emerson make ANNUBAR 285 for measuring gas flow. Its basically a pitot tube. The instrument specifies max flow as 101728 NM3/hr and supplied DP has amax range of 2597 mmWC. How can I decide requisite DP range for measuring a flow of 7000 NM3/hr?

 

Dilip,

Emerson should do (or have done) a calculation. The resulting data sheet (produced by Emerson - not an ISA spec sheet) will show the flow rates a
various DP's, including one for full scale. It is like any other DP flowmeter in the sense that each application requires a distinct calculation to correlate DP with flow.

Based on your info, I would venture a guess: The DP called out (2597 mmWC) is not the full scale (URV) of any of their common capsules, so I SUSPECT... with caveats... that this is the DP corresponding to 101728 NM3/hr. Your Emerson rep would be the best authority.

If my suspicion is correct (it may not be) the turndown you are asking for - about 14-to-1 - will be a challenge for a DP instrument of any kind. You may end up with a full scale DP of a few mmWC. In which case, a smaller full scale capsule (draft range) may be a better fit.

 

Dilip,

Any differential flowmeter follows this simple equation:

Q = K * Square root (DP)

OR, Q1= K * Square root (DP1) & Q2= K * Square root (DP2)

Q1/Q2 = Square root (DP1/DP2)

You know Q1, Q2 & DP1, so DP2 can easily be solved.

Regards,
Ashok Hait

 

ANNUBAR 285 must have a specific flow co-efficient which must be written on the flow calculation sheet that was provided along with ANNUBAR, you can take that into your flow equations to find out the DP range at a particular flow rate but the best option would be to ask EMERSON to do the calc for you.

We also extend services to our customers who buy VERABAR Flow Element (ANNUBAR) from us, as there is no standard for this devices. We use our own internal flow calc based on principles of flow as well as flow coefficient determined through testing.

-Rohit Chandak

 

I'm in agreement with both Mr Rohit & Mr Hait.

Yes, each manufacturer will always have some specific flow co-efficient based on their lab test. However, the general equation given by Mr. Hait will give the rough answers of the range to be selected