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Differential Pressure Level Measurement

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Differential Pressure Level Measurement

Introduction

There are fundamentally two ways to measure level of a fluid in a vessel, which are:

  1. Direct level measurement
  2. Inferential level measurement

Float, magnetostrictive, retracting, capacitance, radar, ultrasonic and laser level measurement falls under direct level measurement technique, whereas weight and differential pressure level measurement comes under inferential level measurement technology.

Differential pressure level sensors or Differential pressure transmitters are probably the most widely employed devices for the purpose of level detection. “Using DP for level is really an inferential measurement. A DP is used to transmit the head pressure that the diaphragm senses due to the height of the material in the vessel multiplied by a density variable.”[1]

Level Detection Using Differential Pressure

Differential pressure level measurement technique makes use of a differential pressure detector which is installed at the bottom of the tank whose level is to be detected. The liquid inside the tank creates pressure which is comparatively higher than the reference atmospheric pressure. This pressure comparison is performed via the Differential pressure detector. A standard differential pressure transmitter connected to an open tank is shown in the figure below.

Differential Pressure Level Detector


In case of open tanks i.e. tanks which are open to the atmosphere, only high pressure ends of the DP transmitter is needed to be connected whereas the low pressure end of the DP transmitter is expelled into the atmosphere. Hence, the differential pressure happens to be the hydrostatic head or weight of the fluid contained in the tank.
The highest level detected by the differential pressure transmitter usually depends upon the maximum height of fluid above the transmitter, whereas the lowest level detected is based upon the position where the transmitter is attached to the tank or vessel.
Now, in cases where tanks or vessels are not open to the atmosphere i.e. in pressurized tanks, both the high and low pressure ends of the differential pressure detector are required to be connected. These tanks are entirely covered in order to avoid release of vapors or steam outside. Due to this, the liquid inside the tank gets pressurized.

Main Features

Key features of differential pressure level measurement include:

  • In cases, where the density of the process fluid is not stable, an additional differential pressure transmitter is employed to evaluate density. This will then compensate for the variations introduced in the process.
  • “To accommodate the measurement of light slurries, differential pressure transmitters are available with extended diaphragms that fit flush to the side of the vessel. However, if the d/p transmitter diaphragm becomes coated, it may require recalibration, which can be impractical and will add to the "cost of ownership".”[2]
  • Differential pressure transmitters are generally found to be more cost effective as compared to other level sensors. However, they often require substantial extra hardware like legs and valve manifolds and labour for successful and stable installation purpose.
  • Differential pressure level sensors are particularly useful for preventing the problem of GIGO i.e. Garbage In, Garbage Out, experienced in various level applications.
  • In actuality, differential pressure level measurement technology does not measure level. It just infers level of the process liquid.
  • Level measurement method with the help of differential pressure is also known as hydrostatic tank gaging method i.e. HTG.
  • Differential pressure transmitters basically operate upon “the principle that the difference between the two pressures (d/p) is equal to the height of the liquid (h, in inches) multiplied by the specific gravity (SG) of the fluid. By definition, specific gravity is the liquid's density divided by the density of pure water at 68° F at atmospheric pressure.”[3]
  • A differential pressure transmitter offers good accuracy over broad level measurement ranges, provided the density of the process remains stable.
  • Various differential pressure cell designs include motion balance, force balance and electronic etc. The same cell designs are used for level detection in pressurized tanks as well as open tanks.

Dry Leg

The weight of the vapors found above the fluid in the tank is considered to be insignificant whereas, the pressure in the vapor area is quite considerable; hence it can not be disregarded and usually transmitted to the low pressure end of the differential pressure cell. This type of pressure connection is known as a dry leg. It is primarily used in situations where liquid vapors are non-corrosive, non-plugging, and have low condensation rates at ordinary working temperatures. “A dry leg enables the d/p cell to compensate for the pressure pushing down on the liquid's surface, in the same way as the effect of barometric pressure is canceled out in open tanks.”[4]  It is recommended to maintain a dry reference leg since buildup of condensate or other fluids have tendency to introduce errors in the level measurement results. A typical closed tank dry reference leg is shown in the figure below.

Closed Tank Dry Reference Leg

 

Wet Leg

In situations where the vapors of the process fluid tend to condense at standard ambient temperatures or happen to be corrosive and unsteady, the reference leg can not be kept dry. It must be then filled with an inert liquid to produce a wet reference leg. For a wet reference leg, it is always suggested that the chosen filling liquid must have a low thermal expansion rate. In case of wet reference legs following two factors should be carefully noted:

  1. The specific gravity of the filling liquid and the height of the reference column are required to be correctly evaluated. Besides, the differential pressure cell ought to be lowered corresponding to the hydrostatic head of the same column.
  2. A sight flow indicator should be mounted above the wet leg in order to get visual indication of the height of the reference leg.

A typical closed tank wet reference leg is shown in the figure below.

Closed Tank Wet Reference Leg


“Either method (wet or dry) assures a constant reference leg for the d/p cell, guaranteeing that the only variable will be the level in the tank. The required piping and valving must always be provided on both the tank and the reference leg side of the d/p cell, so that draining and flushing operations can easily be performed.”[5]

Advantages

Following are the major advantages associated with techniques of level measurement using differential pressure.

  • Differential pressure based level sensors can be easily mounted or retrofitted to the surface of the vessel.
  • To carry out maintenance and testing, these sensors can be provided with block valves for isolating them carefully from the process liquid.
  • They can be easily applied in level measurement applications such as total level in separator vessels where other level measurement devices are not feasible owing to the extensive changes in material formation experienced in the upper state.

Disadvantages

Use of differential pressure transmitters includes few drawbacks too, which are mentioned below:

  • Errors can get introduced in the measurements if the density of the process fluid varies because of reasons such as temperature variations or change of process. Hence, the density of the process must always be maintained constant in order to get accurate results.
  • Differential pressure transmitter works well with clean liquids only. Besides, it necessitates two vessel penetrations for its operation, out of which one is installed near the vessel base where leakage happens.
  • Their use is always avoided with liquids such as paper pulp stock since they result in solidification upon rise in their concentrations.

References

  1. DP Level detection
  2. Differential Pressure Transmitters
  3. DP Level Detector_Principle
  4. Dry Reference Leg
  5. Reference leg for DP cell

Sources

Iceweb

tpub

Controlglobal

Omega