Basics of industrial level measurements 4

All matter occupies space and has some mass and weight. The liquids in a container, tank vessel or column also has weight which is proportional to the density of the liquid. For a same height of say one metre, water with specific gravity of 1.0 will have a weight or head pressure as it is normally referred in technical terms will be more than that of crude oil having a specific gravity averaging 0,65. The volume in the tank and vessel will not have affect as the head pressure is constant per square meter/foot or other units.

This concept of measuring liquid level using its head pressure is very common in the industry and it is equally viable for the level measurement in closed tanks, open tanks, vessels or columns. Where we have pressurized tanks or vessels, differential pressure transmitters are used. This is to compensate or nullify the additional pressure exerted on the liquids. Many different configurations are used for these measurements which include dry legs or wet legs. Also the location of the sensing element will require sometimes elevation of zero, or suppression of zero if the location is above or below the datum line (point of measurement).

For open tanks or vessels, it is quite simple. Any pressure transmitter of suitable range can be connected at the bottom of the tank with proper isolations and the level as it goes up will increase the pressure being sensed which can then be translated into appropriate level signal output with proper CALIBRATION of the instrument.. Here one thing which is to be duly considered is that the wet leg compensation should be considered if the instrument is mounted below the datum line.

Three different scenarios are shown above in the figures. The first one is head pressure measurement in open tank which is mounted at the datum line. Here the head pressure created by different liquids based on their specific gravity will be directly converted into equivalent pressure signal. this will then be transmitted as percentage of level in the tank.

The second figure shows the closed tank level measurement where the tank is having some pressure. Here we need to compensate for this unwanted pressure which will otherwise can cause incorrect output. In this case we use a differential pressure transmitter with appropriate range. the negative leg of the transmitter is connected to the top of the tank so the tank pressure is exerted on both the positive and negative sides, cancelling the effect of each other. Only variable pressure which remains is the head pressure which rises with the liquid level and vice versa.

Here we use two different scenarios, which are using the dry leg on the negative side or wet leg. Dry leg configuration is mainly used in conditions where it is least probable for the liquid to enter the wet leg (negative side of the transmitter), liquid is volatile or quick evaporating etc.. Most of the time wet leg configuration is used so the chances of any liquid entering the negative side is eliminated. This is done by either filling the same liquid fully in the negative leg of the transmitter if the liquid properties allow, or in other cases glycerin or some other liquid is filled to avoid liquid from the tank entering the wet (negative) leg of the instrument. The calculation should be done carefully to evaluate the exact range of the transmitter (setting of LRV & URV) considering the different specific gravities of the measuring liquids and filled liquid in the wet leg. This is normally known as wet leg configuration, because the negative side of the transmitter always remain filled with liquids. One thing which is usually important to consider is that the two liquids should be not reactive with each other in any case. In oil and gas industry, glycerin is a suitable liquid which is quite stable even at higher temperatures and does not easily mixes with most hydrocarbons, like crude oil, furnace oil, different refined product of the crude oil.

Third figure indicates a setup where the transmitter is not located at the datum line. Here there is already a head pressure being exerted on the transmitter capsule or sensing element because of the fluid present in the sensing leg. This pressure elevates the zero reading. To compensate for this, externally we have to shift the zero to suppress this unwanted reading. This phenomenon is known as zero suppression. In olden pneumatic transmitters it was done by adding a suppression spring to the force bar which acted against the pressure and adjusted to suppress the zero to the required value. In electronic devices it is accomplished by adjusting the LRV value.

In this example the tank is again an open to atmosphere type and wet leg configuration is not used here. But if the tank or vessel is closed and pressurized, then here also we will be using the differential type instrument for level measurement, similar to the second arrangement. It should however be noted that since the extra length will be present on both the difference of their specific gravity will decide how much the zero should be suppressed or elevated while considering filled leg length.

In some cases sealed diaphragms are used to measure the level if the liquid is corrosive, toxic or reactive type. if the transmitter is a pressure type and mounted directly with the flanges as shown in the figure bellow, its LRV and URV settings are same as we use in the first case. This is common for open tanks.

The transmitter if uses sealed capillaries, since these have specific liquid with given specific gravity so when these are mounted on a vessel or a tank, these should also be treated as closed tank differential pressure measurement techniques. This is because the sealed liquid has its own head pressure which will vary with varying heights.

Here, location of the top and bottom flanges with respect to the transmitter mounting elevation will be critical in deciding its measurement range. The vendors do normally suggest the best mounting practices as well so if it is followed stringently it simplifies the installation process.

Here we have seen the various instruments which use head pressure or differential pressure to measure the liquid level in most industries. But there are some more techniques employed in the industries to measure liquid or even solid level in silos etc. These will be discussed on basic level in our next few publications on weekly basis.

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