Understanding Dry Leg and Wet Leg Calculation for DP Type Level Transmitters

Introduction:

DP (Differential Pressure) type level transmitters are commonly used in industrial applications to measure the level of liquids in tanks or vessels. One important concept in DP level measurement is the calculation of LRV (Lower Range Value) and URV (Upper Range Value) using the dry leg and wet leg method. In this blog post, we'll delve into the dry leg and wet leg calculation concept and understand how it helps determine the measurement range of DP level transmitters.

1. DP Level Measurement Basics:

Before diving into dry leg and wet leg calculations, let's briefly recap the basics of DP level measurement. DP level transmitters utilize a sensing element that measures the pressure difference between the liquid at the bottom of the tank (hydrostatic pressure) and the reference pressure at the top. This pressure difference is proportional to the level of the liquid and can be converted into a level measurement.

2. Dry Leg and Wet Leg:

In DP level measurement, the "dry leg" refers to the reference side of the transmitter where there is no contact with the process liquid. The "wet leg" represents the side of the transmitter that is in direct contact with the process liquid. By understanding these concepts, we can establish how the LRV and URV values are calculated.

3. LRV Calculation:

LRV (Lower Range Value) is the level at which the output of the transmitter is at its minimum or zero. To calculate LRV, the transmitter is installed with both the high-pressure (wet leg) and low-pressure (dry leg) sides open to the atmosphere. The pressure exerted by the liquid column on the wet leg cancels out the atmospheric pressure on the dry leg, resulting in an output of zero.

4. URV Calculation:

URV (Upper Range Value) is the level at which the output of the transmitter reaches its maximum value. For URV calculation, the high-pressure (wet leg) side of the transmitter is connected to the maximum level of the process liquid, while the low-pressure (dry leg) side remains open to the atmosphere. The pressure exerted by the liquid column on the wet leg creates the maximum output value.

5. Importance of Dry Leg and Wet Leg Calculation:

Dry leg and wet leg calculations are crucial for accurate level measurement using DP transmitters. They establish the measurement range of the transmitter by determining the minimum and maximum values it can accurately measure based on the LRV and URV settings.

6. Zero Suppression and Elevation:

In certain applications, where the lower range of measurement starts above the zero level, zero suppression or zero elevation techniques are employed. Zero suppression involves adjusting the LRV above the actual zero level, while zero elevation raises the URV to accommodate a higher starting point for measurement.

Conclusion:

Understanding the dry leg and wet leg calculation concept is essential for accurately determining the LRV and URV of DP type level transmitters. By correctly setting these values, the transmitter can provide reliable and precise level measurements in industrial processes. Careful consideration of the process conditions and proper calibration are crucial to ensure accurate level readings and optimal performance of DP level transmitters.