Steam pressure sensing line noise
We have been asked by a client to suggest changes to the existing
pressure sensing line configuration to reduce the pressure oscillations
seen in the transmitter output. The transmitters in question are
mounted at almost the same elevation as the taps on the steam line and
are located many feet away horizontally. Moving them is not currently
considered to be an option. The transmitters use a wet sensing leg with
the tubing routed up to a point approximately 15 feet above the
transmitter using a series of vertical and sloped horizontal runs and
then back down to the instrument itself. There are no condensate pots
installed on these lines.
The determination that there are excessive
pressure oscillations generated in these sensing lines (flashing of
condensate is one suggested explaination) is based on comparison with
another transmitter monitoring a similar point which was relocated years
ago such that the line tees off of a 2 foot vertical capped section of
pipe about 1 foot above the top of the steam line and then slopes almost
continuously down from this point to the transmitter.
In all cases the sense lines are 1/2 in stainless steel tubing and are uninsulated after they leave the steam line.
Our
client is looking for some assurance that we understand what is
generating the additional noise and, based on this understandin,
suggested modifications to reduce it short of relocating the
transmitters.
skogsgurra could be right. I would however eliminate the 15ft rise of
the impulse line. Noise could be caused by condensate running back down
the impulse line and burping its way back to the pipe.
Steam flow
measurement hookup is similar to liquid and the transmitter should be
about 2 ft from the taps, lower than the taps with no pockets.
Impulse line is insulated for personnel protection and heat traced if minimum ambient is below freezing.
If
they won't move the transmitter then route the impulse line with a
gradual slope (as low as possible) to a horizontal condensate pot above
the pipe and drop it down to the taps.
First, I'm not exactly clear as to whether this is line pressure (gauge
pressure) or dual impulse lines for flow measurement (DP). The term
"lines" is used, so I'll assume it's DP with dual impulse lines so
that's why there's a rough simulation of an orifice plate shown in the
steam.
Although you described the impulse tubing as a wet leg,
and it does have steam in it, the reason there is no condensate pot is
that there is no way to hold liquid in a condensate pot. The steam that
condenses in the rising leg A (see below) is going to flow by gravity
back into the steam line connection. I suspect that the noise the
customer is seeing is bursts of condensed water reheated to boiling, as
described by SCotsinst & Jim Casey. I'm actually surprised that
there is no complaint of continuing, increasing offset error caused by
condensation in the B leg.
The reason the other situation works is that the entire C leg is a filled with condensate.
Usually
steam flow DP transmitters are installed below the elevation of the
steam line so that the impulse lines can remain filled with
condensate. The alternate installation achieves that by making the C
run an impulse line with the transmitter below the tap point, so the
condensate is trapped.
My suggestion is to create a condensate
trap, fairly close to the steam line. Something like either a pigtail
siphon (pictured below) or a section of pipe replumbed with a vertical
drop to accomplish the same thing (bold pipes below-Instrumentation Ball Valves). The vertical
drop leg trap will hold condensate at elevation D and prevent steam
accessing the entire length of the A riser impulse line. Doing so,
preventing steam from accessing the entire run, will limit the
condensate that flows back into the line.
A filler tee can be
installed so that water can be poured down the A line to fill the
trap. Any excess will just flow back into the steam line.
The closer the trap is to the steam line, the less impulse line remains for condensate to form and flow back into the pipe.
Dan
I want to thank everybody for responding, particularly Scotsinst, Jim Casey and Danw2. Additional information that I didn't provide in my initial posting is that the instrumentation in question consists of gage pressure transmitters and pressure switches monitoring 1000 psi superheated steam in a nuclear power plant. The transmitter signal loops are 4-20 ma current loops, primarily Rosemount 1151s. The instent is to maintain the sense lines to the high points as wet legs by back filling with demin water after maintenance. The responses confirmed our opinion that the excessive pressure oscillations seen in the transmitter outputs by the client are due to condensate flashing in the sense lines. One of the 4 installations was changed years ago to a configuration similar to the working installation diagram provided by Danw2 and this configuration has about 1/3 of the oscillation amplitude seen in the other transmitter outputs. However, as stated in the initial posting, the client is reluctant to make similar changes to the other installations unless there is an absolute guarrantee of significant improvement because of the large cost of running new cable and tubing.