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Naphtha Tank Level Variation w.r.t Day and Night
Naphtha Tank Level Variation w.r.t Day & Night

Hello,

Diaphragm seal capillary DP type measurement envisaged for Naphtha drain oil tank measurement. But, the tank level varies automatically based on day and night. We are not having any temperature and pressure measurement at naphtha drain oil tank.

Kindly suggest how to resolve the level variation.
My email id: bksethi@bhel.in

are all feed and drain valves closed?

You may need to check any sight glasses, or possibly "stick" the tank...

1 out of 1 members thought this post was helpful...

A capillary seal pressure measurement is, by definition, a closed, filled system which is as much a thermometer, reflecting temperature changes, as it is a pressure measuring system. Capillary seal DP level measurements are inherently subject to error due to temperature variation. Even if you had temperature measurements, you'd have to engineer the correction table yourself.

I know of one guy who tried to insulate and electrically heat trace the capillaries and the seal to a slightly elevated, but stable temperature and he managed to get the level deviation on a static, unchanging level down to 6mm, but it was a project (temp sensor location on the capillaries was a problem, I seem to recall he ended up with 7 zones, 2 for each capillary and 1 for each flange/seal, 1 for the process heads on the DP xmtr) and he said next time he'd change level technologies.

Alternative level technologies are
- the ERA system Emerson makes that separately measures the high and low pressures with an electronic connection between the pressure sensors, not mechanical, filled capillary.

- any number of vendors' radar, guided wave or contactless.

Thanks David,

Presently, we do not have any temperature measurement on Naphtha tank. Now, we are trying to take ambient temperature measurement and configure the density correction of Naphtha based on Ambient temperature. Whether this will resolve the level variation or we have to measure tank temperature.

The variation is observed in day/night signifies that it is a ambient temperature related issue. You may cross check the DP transmitter's reading with local level gauge if you have one.

Regards,
RP

Dear RP,

level measurement is constant in local level gauge. Only variation in DP type level transmitter.

The compensated new level=density x actual level. The density to be calculated based on both ambient temperature and pressure or only ambient temperature as the tank is open to flare header.

My question is also whether to take ambient temperature or tank temperature.

Kindly correct me if i am wrong.

Regards,
BKS

The tank temp is the one of major influence.

What is the Level Range and estimated fluid specific gravity in the tank? The liquid temperature is also useful.

Since you are venting to a flare stack, are you using the connections like those used by the level gauge?

> The level measurement is constant in local level gauge. Only variation in DP type level transmitter.

If this is true, then the mass of the naptha in the tank is sufficient that minor ambient temperature changes do not significantly affect its density so that the physical level remains (nearly) the same.

However, even though the physical level remains constant (or shifts insignificantly), the measurement of the level (hydrostatic head pressure through remote seal capillaries on a DP transmitter) does vary significantly with temperature.

The ambient temperature changes the temperature of the capillary fill fluid which expands or contracts with temperature variations and in so doing applies greater or lesser force upon the sensor diaphragm which is interpreted/reported as a greater or lesser pressure. This is an inherent characteristic of closed system capillary measurement.

I am unaware of any formula for correcting reported pressure/inferred level from ambient temperature readings. Maybe someone else does.

A followup query: what is the ambient temp? Some of the high ambients with extreme solar, may require a solar roof or external cooling.

>The tank temp is the one of major influence.
>
>What is the Level Range and estimated fluid specific gravity
>in the tank? The liquid temperature is also useful.
>
>Since you are venting to a flare stack, are you using the
>connections like those used by the level gauge?

The ambient temperature varies from 28 to 47degC. The level range is 0-900mm. The density of naptha is 0.6767gm/cc@15degC.

The connection is simmilar as for level gauge.

It is also observed that during night the level increases (from 4.00pm to 8.00am), and during day the level comes down (from 8.00am to 4.00pm).

>The connection is simmilar as for level gauge.

I don't know what you mean by that.

Does the DP transmitter flange mount directly at the bottom of the tank and use a single capillary remote seal for the vapor space pressure measurement?

Or is the transmitter a dual (Two) remote seal capillary DP transmitter with a remote seal and capillary on the bottom/base and a separate remote seal and capillary at the top?

Which side is connected at the bottom/base? The DP high side or the DP low side?

For both level gauge and transmitter, the connection is 1" flange type.

The DP transmitter is with dual seal capillary type. The tapping for transmitter is on the end side of the tank. The HP side of the transmitter is connected to bottom of tank & LP side is to top.

The capillary also insulated with insulating wool material.

With ambients of 47C, in bright sun, exposed transmitter will experience 75-80C temperatures, that will affect the electronics...

We had to use solar shields, or instrument air purges to keep the electronics within factory specs.

>It is also observed that during night the level increases (from 4.00pm to 8.00am), and during day the level comes down (from 8.00am to 4.00pm).

That is a classic case of unequal heating of the DP capillaries and seal fill fluids.

When the low side is warmer than the high side, the reported pressure differential/level will drive proportionally downwards with an increase in delta T.

When the low side is cooler than the high side, the reported pressure differential/level will drive proportionally upwards with an increase in delta T.

Insulating does not eliminate heat differences, it only creates a lag time in heat transfer.

Good call,

>>It is also observed that during night the level increases
>>(from 4.00pm to 8.00am), and during day the level comes down
>>(from 8.00am to 4.00pm).

>That is a classic case of unequal heating of the DP
>capillaries and seal fill fluids.

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