Saturated steam flow measurement problem

[ghatevinod]

Hello;
We are facing a problem in steam flow measurement. We have a saturated steam flow meter (Orifice based) at plant Inlet. It receives steam from boiler at a press. 8- 12 bar press. Orifice has been designed for 15 Kg/Cm2(g) press. 18,000 Kg/hr flow. This indication is Press Temp. compensated in DCS.

Downstream of this, there is PRV which reduces steam press. to 6.5 Kg/Cm2(g) & there are Orifice-based flow meters at this press. giving flow to Columns.

Main steam flow meter reading shall match to sum of all individual column flow meter, which is not happening. When press is @ 9 Kg/Cm2(g)main steam flow is more by 1300 Kg/hr & when press increases to @12 Kg/Cm2(g) main steam flow meter shows flow more only by 400 Kg/hr. There is no variation in indication of downstream flow meter as they get constant press. of 6.5 Kg/Cm2(g)Down stream flow meters doesn't have any compensation.

We are unable to explain why there is a difference. Pl. help us to understand the same.

 

[TD2K]

As the pressure of the incoming steam increases from 9 barg to 12 barg, the saturated steam temperature will also increase from about 180C to about 192C, that would increase the amount of superheat at the meters operating at 6.5barg but that doesn't seem enough to account for a lot.

You give the differences, what is the total steam flow rates for the high pressure meter and the sum of the lower pressure meters?

Have you checked the flow calculation blocks? I had to troubleshoot a meter issue once, it turned out the flow calculation in the DCS was missing a square root function, it made a slight difference. Although it wouldn't explain the difference you are seeing, it would be worthwhile to verify that line sizes and orifice bores in the field match what was used in the original meter calculations and any other DCS calculations, if those are being done.

 

[georgeverghese]

As suggested, is the 6.5barg steam desuperheated - if so, is there a KOD here ( to knock out excess condensate) before the LP steam meters?

Could there be liquids in the feed to the HP meter - any low points on the supply route?

Impulse lines to meter dp cells all fitted with diaphragm seals?

This is a common problem - not being able to resolve integrator readings between meters in plants - not all get solved.

 

[ghatevinod]

Thanks for the Reply.
We have checked all orifice Calculations. Also the Configuration in DCS. But there is nothing. We don't have any condensate meters.
Superheat at LP meters adds another 200 Kg/Hr Steam. Which doesn't enough to solve our problem. what we have noted is, when the sum of all individual LP steam meter is @ 16000 Kg/hr. (at 6.5 Kg/Cm2 g) HP (Main) Steam flow meter shows 17,200 Kg/hr Flow at 9 Kg/cm2 g
. As inlet press.(HP steam) rises naturally to maintain downstream press. , PRV reduces it's opening. This changes volumetric flow but as down stream flow & press. are constant . Compensation shall calculate exactly same flow rate let us say 17,200 Kg/Hr. but this doesn't happen. as press. rises to 11 Kg/cm2(g) Difference between Individual flow rate sum & Main steam flow reduces to 500 Kg/Hr. & this is where we are unable to understand. Compensation factor shall be more than 1 (& it is above 1) but it doesn't enough big to have fow rate 17,200 again. why this happens. is there any other correction required like density correction. when press. reaches to 13 Kg/cm2(g) there is hardly any difference in both flows.

 

[TD2K]

Let me see if I'm understanding you correctly.

You have 1 inlet HP flow meter, its operating pressure varies from 9 kg/cm^2 g up to 13 kg/cm^2 g and is always saturated steam. You then let the steam down through a pressure reducing valve and have multiple take-offs through flow meters into columns.

I assume as the HP inlet steam pressure changes and its flow rate changes that the sum of the flows through the downstream LP meters is constant?

At 9 kg/cm^2, flow through HP meter is 17,200 kg/hr
At 11 kg/cm^2, flow through HP meter is 16,500 kg/hr as you said the different had decreased to 500 kg/hr over the LP meters.
At 13 kg/cm^2, flow through both HP meter and LP meters are both about 16,000 kg/hr

If I'm right, it's interesting that the flow reduction in the first step is 700 kg/hr for a change in 2 kg/cm^2 and 500 kg/cm^2 for the next 2 kg/cm^2 reduction. If you correct to absolute pressures, a change of 20% in the pressure equals about a 4% drop in measured steam flow.

If you take the HP orifice meter bore diameter and the steam pressure and temperature and the measured dP across the meter (can you get that in the field), do you get the same flow reading as what your DCS is telling you when you do a manual calculation?

 

[ghatevinod]

Thanks for he reply.
We have checked the calculation but are matching to theoretical calculations. Major problem which we are unable to understand is If PRV Outlet press. & Individual column flow remains constant, although PRV inlet press. varies. & as Main steam flow meter is compensated one. then why it's flow varies. PRV Opening changes according to press. Then Inlet volumetric flow varies. But then compensation shall calculate the flow which must be nearer to sum of all individual flow at any condition of inlet press.