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Anonymous Poster #1

Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

06/29/2016 6:56 AM

I am facing one problem in vacuum column level measurement in VV-008 (Slop distillate POT) in CDU/VDU, where VV-008 pressure is -783 mmHg i.e. -10644.96 mmwc. On 24.06.2016 we mounted a new remote diaphragm seal type transmitter replacing the earlier one, with new steam tracing, new glass wool insulation, both upper & lower LT tapings were de choked/cleaned. I entered the following values as LRV(lower range value) and URV(upper range value). LRV: -1620 mmwc URV: -216 mmwc C to C distance(upper and lower tapings distance)is 1530mm. Specific gravity of the liquid, Vacuum Residue: 0.9 At zero conditions i.e. both upper and lower root isolation valves closed, Diaphragm spacer rings drain and vent caps kept opened , then I found PV: -1620mmwc which I had entered as LRV. For URV, -1620+ (1530x 0.9)= -216 mmwc. As I have not considered the vacuum factor for above calculation so i have doubt on my LRV and URV values. There is no LG in VV-008 for cross check, like other sections of vacuum column where we can cross check with the LG. Level Transmitter service is vacuum column slop distillate, Vacuum column pressure -676mmHg. i.e. -9190mmWC. Kindly enlighten me about how to calculate LRV and URV values in vacuum column level transmitter.

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Guru

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#1

Re: Vacuum column Level measurement in VV-008(Slop distillate Pot)

06/29/2016 7:36 AM

What is the capillary fill fluid and what is its specific gravity ?

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Power-User

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#2
In reply to #1

Re: Vacuum column Level measurement in VV-008(Slop distillate Pot)

06/29/2016 8:08 AM

1) Is the bottom tap the datum, the zero reference point for 'level' or is the level zero reference point above the bottom tap?

2) The high and low taps are 1530 mm elevation distance apart.

What is the maximum liquid level from the bottom tap ? It can't be more than 1530mm, it is less?

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Participant

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#8
In reply to #1

Re: Vacuum column Level measurement in VV-008(Slop distillate Pot)

07/01/2016 6:48 AM

silicon oil.

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Venezuela - Member - New Member

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#3

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

06/29/2016 10:22 PM

Friend,

I remember some compensation for Dp level calculation on boiler drum applications, in your case I don´t remember any additional compensation for Dp level calculus on vacuum applications.

There is a manufacturer with good recommendations for Dp level installation on vacuum conditions, just take a deep look at these documents.......

http://www2.emersonprocess.com/siteadmincenter/pm%20rosemount%20documents/00840-0100-4016.pdf

http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Documents/00809-0100-4002.pdf

What is your process temperature? Why are you using steam tracing? Is the process fluid highly viscous? or Do you have low ambient temperature???

By the way, I suggest calculate the range similar to others typical application

I see a mistake in your calculus, kindly please revise it.......

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Participant

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#9
In reply to #3

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/01/2016 6:57 AM

Kindly correct my calculation. although specific gravity is 0.9 at 15degc, but it gets reduces at operating temperature of 389 degc i.e will be 0.845. so URV value will be slightly changed. But main issue is that level transmitter PV is not changing either way if MV changes. As is is viscous liguid so we have used glass wool insulation, steam tracing . LT can work upto 400 degc as per specifications. really frustrating.

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Guru
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#4

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

06/30/2016 2:57 AM

<... -783 mmHg...> Given that 1 atmosphere is 760mmHg, That is where the first problem lies.

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Participant

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#7
In reply to #4

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/01/2016 6:43 AM

Vacuum distilation pot(VV-008) PT is malfunctioning..it cannot be -783mmHg. Vacuum distillation column pressure is -676mmHg. The installed transmitter can work upto process operating temperature range 400degc. transmitter is not responding to MV change at all. High side of the transmitter is installed at lower tapping and lower side installed in the upper tapping, like normal level transmitter. This vacuum level transmitter is really frustrating, as it not responding at all, after so much of hard labour. LT mounting ,impulse line was lowered by about 1mtr from high tapping . But, same result, transmitter is not responding to MV change at all. capillary tube liquid Slicon oil.

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#10
In reply to #7

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/01/2016 12:04 PM

Personally, there are a couple of things not clear on your application, you mentioned at first post a remote diaphragm seal type transmitter and then you mentioned impulse line, where is this impulse line installed?

For me Silicone oil can have several compositions, I mean DC 200, 704, 705.....

Please, Can you post a couple of photos of your transmitter installed?

Do you can tell us the transmitter manufacturer and complete model including the remote seal numbering?

How sure are you about real level if you don´t have LG?, You did some test opening and closing the root valve and checking at the drain if any liquid came out......

I suggest you first evaluate if your transmitter model can be used on vacuum application according your pressure and temperature indicated (-676mmHg, 389 degc)

and,

Do some basic checks with the transmitter installed, I mean put pressure on its high and low pressure ports using the spacer ring drains in order to check how is responding the transmitter......

What do you mean with MV change?

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#5

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

06/30/2016 12:49 PM

The OP is all concerned about the vacuum pressure and other things, but the vacuum pressure has nothing to do with the transmitter setup. It is irrelevant.

The transmitter's exact LRV and URV can be calculated from the following data:

Elevation between taps: 1530mm

Zero level elevation datum above the bottom tap centerline: ?

The 100% maximum level above zero level datum or bottom tap (need to specify which): ?

Medium Specific Gravity: 0.90

Capillary Fill Fluid Specific Gravity: ?

Only two out of five required data are available, and if true to form where the OP never replies, neither he nor we will ever know what the calculated LRV and URV are.

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#6

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

06/30/2016 2:30 PM

It cancel's the static pressure whether vac or pressurised

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Guru

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#11

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/03/2016 2:20 PM

You have not revealed

- the exact configuration of the transmitter, model or brand

- the installation data needed for calculating the LRV and URV (as requested by Carl and me)

but, I'll comment nonetheless.

1) It is unlikely that the transmitter can operate at 400 Deg C

>”The installed transmitter can work upto process operating temperature range 400degc.”

I stongly doubt that the statement is true.

Rosemount publishes a comprehensive fill fluids specification guide, available here:
http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Documents/00840-2100-4016.pdf

The nature of the business is that the instrument manufacturers and the vendors who specialize in constructing remote seal assemblies all use the same fill fluid products, so the specs Rosemount provides are valid across the board, regardless of the vendor. (DC is Dow Corning, the fluid manufacturer).

You might have misinterpreted a statement, like the one below, regarding temperature limits:

The maximum temperature stated is for positive pressure service, not for vacuum service. Vacuum service temperature limitations are typically determined by a temp vs vacuum pressure graph/chart. Rosemount's chart of operational limits for vacuum at process temperature for DC 705, a typical high temperature fill fluid, is below:

Note that the chart scales are mislabeled, the units on the left are PSIA and the units on the right are mbarA, so the top of the chart is atmospheric pressure, the bottom of the chart is high vacuum approaching absolute zero. Also note that the horizontal graph grid is aligned to the PSIA values, not the mbar values on the right. For mbar values there are tick marks at each value that unfortunately do not reproduce well in the graphic image.

As the vacuum level increases the operational process temperature limit decreases.

For instance, hard vacuum pressure near absolute zero pressure (the bottom of the chart) can only tolerate temperatures to about 260 to 270 Deg C.

350 deg C service is limited to vacuum of 350 mbarA or approx – 650mbarG

400 deg C is not even on the chart for this fill fluid.

That's why I asked what fill fluid is used.

The reason for these limitations is that the fill fluid 'boils' or flashes to a gas state at certain pressure and temperature points. A gas bubble in a capillary tube represents an increase pressure within the tubing that does not reflect the pressure applied to the diaphragm, which is another way of saying 'pressure measurement error'.

In extreme cases, the pressure in the capillary will either force the diaphragm outwards, distorting the diaphragm, or even burst the diaphragm. A distorted diaphragm never regains its original shape or flexibility and will produce measurement errors afterwards. A burst diaphragm creates a zero pressure reading on the associated port and results in the making the transmitter dysfunctional. These are well-known limitations of remote seals in vacuum/high temperature applications, not unique to any particular vendor.

2) LRV URV

The graphic below shows the calculation for LRV/URV for capillary remotes seals for level measurement.

https://www.honeywellprocess.com/library/marketing/tech-specs/34-ST-03-64.pdf (page 20 pdf)

I've used assumed values for those not provided. You can re-calculate using your secret values.

SGf value - specific gravity (SG) of remote seal fill fluid; DC 705 SG = 1.09

SGp value - specific gravity of the process liquid = 0.90

d value - Elevation distance between taps, the lower seal (high side) and upper seal (low side) = 1530 mm

b value - distance between bottom tap and maximum level height (what 20mA represents) = 1500 mm

a value - distance between bottom tap and minimum level (what 4mA represents) = 0 mm

1) Calculate the LRV

LRV = (SGp * a) - (SGf * d)

At min level (empty), there is 0 process liquid pressure on the high side, so

a = 0

SGp = 0.90

(SGp * a) = 0 mm

The fill fluid exerts a (negative) pressure of (SGf * d) or (1.09 * 1530 mm) on the low side

(SGf * d) = 1.09 * 1530 mm = 1667.7 mm

LRV = (SGp * a) - (SGf * d)

LRV = 0 mm – 1667.7 mm = -1667.7 mm

LRV = -1667.7 mm

2) Calculate the URV

URV = (SGp * b) - (SGf * d)

At the maximum fluid level, there is a pressure of (SGp * b), or (1.09 * 1530 mm) on the high side.

(SGp * b) = 0.90 * 1500 mm = 1350 mm

URV = (SGp * b) - (SGf * d)

1350 mm – 1667.7 mm = - 9.48"

URV = -317.7 mm

3) Transmitter’s Range

LRV = 4mA = -1667.6 mm.

URV = 20mA = -317.7 mm.

The transmitter's range is -1667.7 mm to -317.7 mm., a span of 1350 mm., which is the head pressure of 1500 mm of liquid with a SG of 0.90.

Those are the LRV and URV values, but whether the transmitter will function or for how long at high process temperature temperature is an open question.

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Guru

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#12

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/03/2016 2:52 PM

The app note at the link below has a graph showing the liquid vs vapor state of fill fluid at temperature vs pressure.

For those of you dealing DP level under vacuum service, it is well worth the read.

http://www2.emersonprocess.com/siteadmincenter/pm%20rosemount%20documents/00840-0100-4016.pdf

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Guru

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#13

Re: Vacuum Column Level Measurement In VV-008(Slop Distillate Pot)

07/03/2016 2:57 PM

How embarrassing. Note correction to post #11

2) Calculate the URV

URV = (SGp * b) - (SGf * d)

At the maximum fluid level, there is a pressure of (SGp * b), or (1.09 * 1530 mm) on the high side.

(SGp * b) = 0.90 * 1500 mm = 1350 mm

URV = (SGp * b) - (SGf * d)

1350 mm – 1667.7 mm = - 9.48"

URV = -317.7 mm

Should read:

1350 mm – 1667.7 mm = - 317.7 mm

URV = -317.7 mm

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