Magnetorestrictive Level Transmitter issues

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Thread Starter

Braindead

I work on several Deepwater offshore production facilities. We are having two different but still related problems with our magnetorestrictive transmitters located in extreme high vibration process areas.

First we occasionally lose signal for no apparent reason back to the control system from our magnetorestrictive level transmitters attached to bridles/chambers off the side of our vessels. We have tried several different manufacturers of transmitters, debounce timers, different ways of mounting and other common practices. It is not a wiring or input card issue. We have high loss of end devices. We know it is attributed to vibration. We are looking for recommendations for a more robust/resilient transmitter that can handle this extreme vibration without having to modify our bottom flanged design on our chambers if possible (originally made by k-tek and jogler. Anyone have any experience in extreme vibration and can recommend anything?

Next we have magnetic floats in these chambers. We were recently told the floats may be losing magnetism and causing issues, personally I have never heard of this and wonder if anyone else has run into this type of issue.
 
you need to be a bit more specific about how your instruments are wired, i.e. te terminations, wire, etc.

have you measured the vibration levels using the handheld probe for motors in your maintenance shop?
 
for reference when the vibration levels exceed 1 G continuously, you'll start impacting the reliability of you electronics and mean time between failures.

at 5-10 G's well --- at 10 HZ and and a 1 mm amplitude of displacement you are approaching 400 G's, at that level you'll have catastrophic failures that may not be confined to the electronics.

Get your vibration tech involved, post haste, there are few escape route off a platform when you lose process containment
 
G's = (2 pi freq)^2 displacement/g, g=9.8 m/s2, so for a displacement amplitude of 0.1 mm, and a frequency of 100 Hz, the acceleration =4 G's.

Most electronic modules require lover vibration levels for reliability. Above 20 G's you start having catastrophic not just fatigue failures.
 
Hi,

The FPSO's I worked on made use of Guide Wave Radars for their interface measurements. I have some experience on the magnetorestrictive transmitters and I must say I don't like it.

If you have a lot of vibration I suggest you use the cable type of Guide Wave Radar to add some flexibility to the transmitter's probe but the GWR is not influenced by anything, not even vibration since it operates at the speed of light.

The Magnetoristrictive transmiter have a solid rod so the vibration will influence that directly and the vibration will therefore be transferred to your magnetic float. Also keep in mind that if you want to de-magnatize a magnet you hit it with a hammer a couple of times so the vibration and constant shocks due to the vibration have a similar effect on your transmitter's measuring float magnet. I believe that these types of transmitters are just good for ideal conditions like diesel tanks onshore. They are sometimes used to also measure the water at the bottom of these diesel tanks so they can be used for interface measurements but personally I will never try and use one of them in a offshore environment to measure the interfaces in these high pressure, high velocity vessels.
 
Hi Sam,

I am reading your comments about interface level tech for crude and water + emulsion.

First, thank you for your posts, you know what you are saying.

I would be pleased if you can explain, as you said: "Do not use ANY type of DP instrument, the explanations why not is to long so just trust me on this."

Can we trade some experiences?
Is your crude density very different from produced water? Do you have emulsion on your system? do you use the GWR in a standpipe, or inside the vessel?

Do you use GWR in both separators?

Thank you for your reply.

If you like to contact me directly, please mail to [email protected]
 
Hi Mateus,

there is only one instrument when you are working offshore on a FPSO or Platform that will give you proper and reliable interface measurements and that is a GWR. We have tried various things in the past and everything else has limitations and problems.

If you are want you can go through the same exercises we did and try and use a DP Tx or a capacitance probe and you will find that even if you manage to do the calibration calculations for a DP transmitter, which is about two pages long, and even if you know how to install a capacitance probe in an open vessel, offshore is a different ball game all together. Stick to the GWR in offshore high pressure and velocity vessels. The GWR's should be installed in bridles next to the vessel and should be slightly higher at the top and slightly lower at the bottom than the vessel with overlapping sight glasses. There should be two bridles, one for the DCS and one for the ESD and one for the DCS transmitters. Some design engineers sometimes believe that it is a bad idea to install the same type of transmitter but it is not. Installing a capacitance probe and a GWR on one vessel is a bad idea since in case the capacitance probe fails it is impossible to recalibrate a new one on a live vessel other than to just set it up according to the readings on the GWR. Like I said, stick to the GWR on all the pressure vessels offshore and your trips will be a breeze. On one vessel we had a 98% up time because everything was done like this so I am talking from practical experience. On other vessels where some bright spark design engineer has decided to make the bridles only half the size of the vessels we were called out three four times a night.

As to your other question, produced water is much the same as clean water with a sg of about 0,97 while crude have a density of about 0,84. On your produced water, heating and cooling medium vessels your can do wet legs, they work the best. In all these applications capillary DP Tx’s will work even better. Dry legs are only done on open vessels so stay away from that on any closed crude or condensate pressure vessel.
 
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