DP Remote Seal Transmitter

Well there's a few possible reasons here for these differential pressure reading differences. But from your information, any of these could be your root problem.

First your transmitters are no longer accurately transmitting the actual vacuum pressure that they are observing. From what you've told us I cannot say if this is your problem but I personally doubt that this is your problem.

Next, you have to examine the actual differential pressure difference you have observed (numerical values are so important) and compare the tolerance (error) range of these instruments. Far too often people forget that the measurement tolerance range is a percentage of the maximum value that can be measured. At the low end this tolerance window will be a large percentage of the actual measurement.

Lastly, you might actually have a real vacuum pressure difference that these instruments are reporting. These "towers" maybe actually performing differently than they did at commissioning. Possibly the process happening inside these towers has changed the accuracy of these instruments in some fashion. (The region next to the sensor is accurately being reported but this region's pressure is no longer the same as your critical chamber.) It amazes me how often people blame the instruments instead of believing them.

You've established that the operating pressure range is 1 torr (1mmHgA). That's impressive.

Over what temperature range does this process run? (Degrees celcius)

To my knowledge, all industrial DP transmitter manufacturers provide a chart similar to this one (Honeywell), that provides working temperature ranges over which a remote seal can function at various pressure exposures.

Note that Honeywell's DP transmitters work to specification only above 26-27 torr with special vacuum construction and up to 175°F (~80°C)

Have you checked Hitachi performance chart for (vacuum) pressure vs temperature vs fill fluid to establish what it's expected preformance is?

I agree with your statements and woud like to add that this "limitation" in operational accuracy and performance is possible due to out gassing of the fill fluid in the closed system. In the fitting and filling process the DP Tx and the seals are evacuated under high vaccum conditions and then the fill fluid, normally Silicone oil is sucked in to the evacuted Tx body and seal and capilaries. During the evcuation process the fill fluid is degassed to remove the suspended air as if this is not done it will change the compressabiliy of the fluid under pressure. In vacuum conditions the fluid in the closed loop can again start out gassing if it was not done correctly during the filling proceess or if the vacuum is below the fluids specified working range . I suggest that you look at an alternative principle to measure the level such as Radar. Good luck Rudy.

Hi Mr.Iris

Thanks for your reply.As per manufacturer(Hitachi)their transmitter can work even

below 1 torr.Temperature at the bottom of the tower will be around 24 to 30 degree centrigrade and top side 130.

Dear KMM,

You may be better installing absolute pressure transmitters and subtracting the readings in your control system. DP transmitters don´t like such high vacuum.

Regards,

emlynsee

Hello Mr.Emlynee Thanks for your reply

You are right,DP transmitters will not like high vacuum.The manufacturer Hitachi assured that it can work at even less than one torr.We are planning for a shutdown to inspect the diaphragm.

Is Hitachi's spec at "less than 1 torr" for remote seals or for a basic style, pipe/tubing-plumbed DP transmitter?

Did Hitachi install the remote seals? Or did a 3rd party?

Is the remote seal transmitter all-welded construction?

Mr.Iris

Thanks of your reply.It is a remote seal diaphragm transmitter.

The transmitter was installed by a third party.

On google search for the model no.EDR-N7S , It seems the manufacturer used latest

technology to provide good response at vaccum service.

Regards

kmm_mohideen

Today we break the vaccum in the tower and the difference between the LT comes

down to4 to 5%.At vaccum the difference around 40%.It seems the sensor not

responding well at vaccum.

We will be inspecting the sensor diaphragm tommorow.

To inspect the sensor you will need to break the seal. Before you do that you should try and check the calibration with the chemical seals still attached. Also once you have removed the seals you will need to have them refilled and unless you have a filling rig and have done this before, I would recommend that you rather have the OEM do so.

Iris also raised a very valid point in that on vacuum sustems it is always good to have a fully welded system. If this was done then you will not be able to remove the seals.

To many times people try and fill the capillaries and seal on their own or have some gauge company do it for them. Gauges are not as sensitive to some air being in the fluid as electronic DP Transmitters and thus need to be filled differently. Endress+Hauser have a program to calculate the actual fill volume needed and also the effect that temperature will have on the accuracy based on the actual on site conditions.

Have you considered any alternative level measurement?

Many Greetings Mr. Rudy Rudolph

You are right,we are not breaking the seal.Just inspected the capillary and diaphragm for any physical damage.All seems to be ok.

We are trying to conduct scanning of the tower(by internationally reputed firm by name TRACERCO diagnostics),to have knowledge about exact level of the tower.

Thanks and Regards

The performance chart is VERY impressive, if it's true.

Please keep us informed of your progress in solving this issue.

Greetings to All,

When we inspected the diaphragm,no abnormal.We removed both the transmitters

applied equal pressure and vaccum, results are fine.

The test was carried out at room temperature,not at site condition.Further we are investigating.

We have fixed two new transmitter and found zero stability.The level of the tower

was increased and reading found matching till 80%.we start emptying and found 4 to 5% deviation between the transmitter.

We are further investigating the performanceof the new set of transmitter.

Thank you, Mr Mohideen, for updating us.

Agreed I thank you for updating us.

While it is certainly understandable to investigate any and all unexplained conditions I just wanted to emphasize that this vacuum anomaly maybe a normal condition in the first place. Depending on the kinematics and secondary chemical reactions of your chamber this deviation may just be normal. Listen to the team your employer has hired to explain what has happened. Bring their conclusions to us and we will happily debate their conclusions. But I wish to again emphasize that the differences from your expectations, the resolution of the monitoring instrumentation and the unknown mechanics of your system makes it difficult to offer proper guidance.

Greetings to all

Our specialist team analysed the problem and comeout with a solution.

The different in reading is due to calibration of the transmitter at different temperature and pressure(vacuum)from the operating condition.Operating at 1 torr and temperature 20 to 25 deg centigrade.

It was decided by the core team to keep at vacuum condition and adjust zero and will check zero stability.

Greetings to all,

Finally we received reply from the licensor.It seems the operating parameter of the

tower is not as per original design.Only in one tower the operator maintain correct parameter.

Inlet flow must be less than the blow down from the tower.Hope we find the root cause.But now the complete plant is under shutdown.When once started we can implement the recommendation from the licensor. Thanks to all,who participated in the discussion.