Level measurement systems that use differential pressure ΔP as the sensing method, are by their very nature affected by temperature and pressure.
Recall that the measured height H of a column of liquid is directly proportional to the pressure P exerted at the base of the column and inversely proportional to the density ρ of the liquid.
H α P/ρ
Density (mass per unit volume) of a liquid or gas is inversely proportional to its temperature.
ρ α 1/T
Thus, for any given amount of liquid in a container, the pressure P exerted at the base will remain constant, but the height will vary directly with the
H α T
Consider the following scenario. A given amount of liquid in a container [figure (a)] is exposed to higher process temperatures [figure (b)].
As the amount (mass) of liquid does not change from figure (a) to (b), the pressure exerted on the base of the container has not changed and the indicated height of the liquid does not change. However, the volume occupied by the liquid has increased and thus the actual height has increased.
The above scenario of figure is a common occurrence in plant operations. Consider a level transmitter calibrated to read correctly at 75 DegC.
If the process temperature is increased to 90 DegC, the actual level will be higher than indicated.
The temperature error can also occur in wet-leg systems (figure c).
If the reference leg and variable leg are at the same temperature that the level transmitter (LT) is calibrated for, the system will accurately measure liquid level. However, as the process temperature increases, the actual process fluid level increases (as previously discussed), while the indicated measurement remains unchanged.
Further errors can occur if the reference leg and the variable (sensing) leg are at different temperatures. The level indication will have increasing positive (high) error as the temperature of the wet reference leg increases above the variable (process) leg.
As an example, consider temperature changes around a liquid storage tank with a wet leg. As temperature falls and the wet leg cools off, the density of the liquid inside it increases, while the temperature in the tank remains practically unchanged (because of a much bigger volume and connection to the process). As a result the pressure of the reference leg rises and the indicated level decreases. If it happens to the boiler level measurement for a shutdown system it can even lead to an unnecessary reactor trip on boiler low level. However, high-level trips may be prevented under these circumstances. In an extreme case the wet leg may freeze invalidating the measurement scheme completely, but it could be easily prevented with trace heating.
False high level indication can be caused by an increased wet leg temperature, gas or vapour bubbles or a drained wet leg.
A high measured tank level, with the real level being dangerously low, may prevent the actuation of a safety system on a low value of the trip parameter.
The real level may even get sufficiently low to cause either the cavitation of the pumps that take suction from the tank or gas ingress into the pumps and result in gas locking and a reduced or no flow condition. If the pumps are associated with a safety system it can lead to possible safety system impairments and increased probability of resultant fuel damage.
Effect of Pressure on Level Measurement
Level measurement systems that use differential pressure ΔP as the sensing method, are also affected by pressure, although not to the same degree as
Again the measured height H of a column of liquid is directly proportional to the pressure P exerted at the base of the column by the liquid and inversely proportional to the density ρ of the liquid:
H α P/ρ
Density (mass per unit volume) of a liquid or gas is directly proportional to the process or system pressure Ps.
ρ α Ps
Thus, for any given amount of liquid in a container, the pressure P (liquid pressure) exerted at the base of the container by the liquid will remain
constant, but the height will vary inversely with the process or system pressure.
H α 1/Ps
Most liquids are fairly incompressible and the process pressure will not affect the level unless there is significant vapour content.