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-Regulators) 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 custormer 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).   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

We had excessive HF oscillations on a pressure transmitter monitoring air flow in a dryer. We put an pneumatic RC filter into the sensing lines with great success.Later it was find out that the oscillations came from mechanical vibrations of the entire dryer.

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