Steam orifice meter and insulation

[pratim49]

We have medium pressure steam supply to a plant with two parallel orifice flow meters installed side by side* say FT-101 and FT-102. Both lines are insulated upto impulse tubes.

Usually 101 is kept in line and shows flow around 40-45 T/H. Recently we have replaced orifice of FT-102* removed its insulation* and took it in line. For seven days* the meter shows correct readings* reconciled by other meters. Yesterday* the insulation was replaced. Immediately* FT-102 starts showing reduction in flow readings in the tune of almost 10%. As we again remove the insulation* FT-102 starts showing correct readings.

We fail to understand the relation between insulation and flow reading. Could you please provide any feasible cause for this?

[amshah]

use condensate pot with filled water no need to nave insulated tubing or piping.

[carl_e]

Presumably the DP transmitter is mounted below the elevation of the steam line* with impulse tubes that descend some distance.

Sometimes a pair of condensate pots is installed at the upper elevation near the pipe taps. The purpose of condensate pots is to maintain a constant level of condensate in the impulse lines to transmit the hydrostatic pressure to the transmitter at a reduced temperature. The level of condensate is maintained by allowing any excess condensate to flow by gravity back into steam line. If there are no condensate pots* there should be a Tee at a high point for filling the impulse lines with water and venting air.

The condensate pot and impulse lines are not insulated in order to dissipate heat in order to condense the condensate to liquid. The purpose of impulse tubing insulation is to cover the heat trace when the installation is subject to freezing temperatures and heat trace is needed to prevent ice formation in freezing weather.

Whether there are condensate pots or not* the level of the condensate in the impulse lines must maintain a constant level to avoid a zero shift error.

A low DP flow reading can be caused when
- the condensate level in the low leg is not at its nominal level (lower than expected)* or
- the equalizing valve used for checking zero (separate valve or in a 3 or 5 valve manifold) is leaking slightly by lowering the high side reading.
- the lack of a drain hole in the orifice plate can cause upstream condensate buildup which results in a lower dp for a given flow
- density of water in the impulse tubes differs due to uneven temperatures of the impulse tubing . TuV NEL's Good Practice for Impulse Lines for DP Flow Meters reports that a 2m liquid head of water between the transmitter and the pipe where there is a temperature difference of 10°C between the impulse lines would report an error of 49Pa (0.2"wc).

Is insulation likely to affect whether there is a leak in the drip/drain at the bottom of the impulse tubing or through a blow-down valve on the low side? Probably not.

Is insulation likely to affect a leaking equalizing valve? Would insulating the valve cause it to leak* but not leak when not insulated? I doubt it.

Is condensate backing up due to a lack of a drain hole? Pipe insulation might affect the level of condensate backing up upstream of the orifice plate* but would impulse tubing insulation affect it? I don't know.

Could your reported error of 10% be caused by a difference in the density of the water due to a temperature difference between impulse lines? I doubt it* but you haven't revealed the numbers.

What could cause a drop in the level of the condensate in the low leg* that would influenced by insulation?

Condensate can flash back to steam
- when the condensate level in the piping close to the pipe or in a condensate pot is very close to the steam piping so the heat conducted through the impulse piping keeps the condensate near saturation temperature.
- during pressure swings
- when there is superheat available
- when a leak in a condensate pot vent or filling vent at the top elevation of the impulse tubing allows steam to flow through when it should be deadheaded* with no-flow. The heat from steam flow can keep the condensate at a high temperature where it can flash back to steam.

[nutcha]

Very details and good explanations.