Brain Teaser: Problem Identification & Solutions

I see lots of problems and mistakes on DP level. Here is a classic DP level problem that I've encountered. This one is hypothetical since I don't have the data on the originals.

What are the biggest problem(s) with this setup?

The biggest problem would be that the reference leg of the DP transmitter is not at the reference pressure exerted on liquid being measured (since transmitter is vented to a pressurized compartment, sealed from the outside ambient air). So, whenever the compartment pressure changes, it would cause a direct change in DP reading and indicated level change. 

What should an acceptable margin of error be for this scenario as compared to a mechanical float/tape measurement, and why?

Exact values would be debatable, but it is safe to say that compartment pressure should not change by more than 0.5 psid. Based on that, and a ratio of 27.71 inches H2O per psid, we could potentially see up to 13-14 inches of level error due to compartment pressure changes (but our ears would definitely be popping). 

Think about some possible solutions to the biggest problems noted above, but also emphasize the BEST solution(s).

Best solution - run a tube (or better yet, a capillary & diaphragm) through the wall to the actual ambient air pressure exerted on the surface of the tank liquid being measured. Lots of ways to do this, but until the reference tap of the DP transmitter is accurately referencing out the true ambient pressure, there will be errors in the reading. 

Note - if this was reading a level with a small span, it could be severely problematic since 13-14 inches of potential error could be outside our safety margins or control error envelopes. 

What DP would an instrument tech need to apply to test the 0, 50, and 100% test points of the transmitter?

Since the tap is 9" above bottom of the tank,, that leaves 18 inches above it, with SG = 1.03.

0% point DP would equate to 18 x 1.03 = (18.54 inches dp) at 0%

The 50% point would add half of the span to that: 0.5 x 15ft x 12 inches/ft x 1.03 = 92.7" +18.54" = 111.24 inches DP

The 100% point would add the full span to that: 1.0 x 15ft x 12 inches/ft x 1.03 + 18.54" = 203.94 inches DP

NOTE - when doing the calibrations on this system with the reference tap design flaw, it would be very challenging to get any meaningful data. The calibration would change by several inches each time ventilation system is adjusted and whenever access hatches are opened. 

If the current level was at 90" above the 0% point, what would the transmitter mA output be (assuming 0-100% = 4-20mA)?

90 / (15ft x 12in/ft) = 0.5 which equates to 50% - so 12.0mA

*Note - we didn't need to factor in SG on the above calculation because it factors into the actual value and the span value proportionally (transitive property of multiplication or something like that..)

Assumptions:

The DP transmitter is calibrated to measure the 15 ft span with 0% being indicated when fluid level is 27 inches above bottom of tank, and 100% being indicated when level is 27 inches below top of tank.
The fluid inside is seawater with consistent SG = 1.03. The system is at sea level.

Assume the measurement port connection is a flange / diaphragm & capillary type and is setup and working properly and for simplicity the diaphragm is level with the DP transmitter. The DP transmitter tap & flange/diaphragm is at an elevation of 9 inches above the bottom of the tank.

Fans are controlled manually by operators, and the compartment pressure varies based on number fans running.

Has anyone seen issues like this in your plants?