Serge Vessel – Level Control

S.R. Bhat (Technical Director, COMFIT)

Many times, a product produced in a process equipment is used as feed to another equipment. In these cases, it is necessary to make the feed flow to the second equipment steady, i.e. any variations in the feed should be very slow. This must be achieved even if the product out coming put of the first equipment is not steady. E.g. Naphtha coming out of the crude distillation unit of a refinery is unstable, i.e. it contains a lot of volatile components. These volatile constitute Liquefied Petroleum Gas (LPG), which is very valuable. This stream is fed to another distillation column called Naphtha Stabilizer. Purpose of the Naphtha Stabilizer are two. They are:

1. Recover the LPG, allowing minimum slippage of LPG to Naphtha.

2. Remove LPG from Naphtha with minimum slippage of Naphtha to LPG. To achieve this, a serge vessel is introduced in between the Crude Distillation Column and the Naphtha Stabilizer. Feed flow into the stabilizer is controlled by a control valve introduced between the serge tank and stabilizer. Process variable for this control loop is the liquid level in the serge tank. This level keeps varing around 50%. If this controller is well tuned, level will always mainted tightly near the 50 % mark. This would defeat the purpose of surge vessel by making the vessel outlet flow closely follow the badly varing inlet flow. On the other hand, by keeping the controller sluggish (i.e. making proportionality gain low), the surge tank serves its purpose. When the input flow to the serge vessel increses, sluggish level controller acts very slowly, resulting in the slow and steady increase of outlet flow from the vessel. This increses the level in the serge tank. On the other hand, when the inlet flow to the serge vessel reduces, exactly the reverse happens.

This is a strange case of untuned controller gives the desire result. This has been successfully done in the Naphtha Stabilizer of refinery, which resulted in substantial increase in of the production of the valuable LPG, but also improved its weathering.

Serge Vessel – Level Control

S.R. Bhat (Technical Director, COMFIT)

Many times, a product produced in a process equipment is used as feed to another equipment. In these cases, it is necessary to make the feed flow to the second equipment steady, i.e. any variations in the feed should be very slow. This must be achieved even if the product out coming put of the first equipment is not steady. E.g. Naphtha coming out of the crude distillation unit of a refinery is unstable, i.e. it contains a lot of volatile components. These volatile constitute Liquefied Petroleum Gas (LPG), which is very valuable. This stream is fed to another distillation column called Naphtha Stabilizer. Purpose of the Naphtha Stabilizer are two. They are:

1. Recover the LPG, allowing minimum slippage of LPG to Naphtha.

2. Remove LPG from Naphtha with minimum slippage of Naphtha to LPG. To achieve this, a serge vessel is introduced in between the Crude Distillation Column and the Naphtha Stabilizer. Feed flow into the stabilizer is controlled by a control valve introduced between the serge tank and stabilizer. Process variable for this control loop is the liquid level in the serge tank. This level keeps varing around 50%. If this controller is well tuned, level will always mainted tightly near the 50 % mark. This would defeat the purpose of surge vessel by making the vessel outlet flow closely follow the badly varing inlet flow. On the other hand, by keeping the controller sluggish (i.e. making proportionality gain low), the surge tank serves its purpose. When the input flow to the serge vessel increses, sluggish level controller acts very slowly, resulting in the slow and steady increase of outlet flow from the vessel. This increses the level in the serge tank. On the other hand, when the inlet flow to the serge vessel reduces, exactly the reverse happens.

This is a strange case of attuned controller gives the desire result. This has been successfully done in the Naphtha Stabilizer of refinery, which resulted in substantial increase in of the production of the valuable LPG, but also improved its weathering.

Mastering Volatile Liquids Tank Level Measurement

S.R. Bhat (Technical Director, COMFIT)

Your approach to solve the level measurement issue in refrigerated tanks for volatile liquids is both intriguing and creative. Let's break down the concept and application:

1. Current Issues with Refrigerated Tank Level Measurement:

o Displacement and radar transmitters encounter issues like high cost, complicated calibration, difficult installations, and additional chamber requirements which not only add to the cost of installation but also increase the load on refrigeration system.

o Remote seal transmitters struggle at low temperatures, which can lead to frozen seals and their total failures.

2. Challenges with Traditional DP Transmitters:

o Installing DP transmitters at the bottom risks exposure to very low temperature fluids, leading to failure of the transmitters. This shall cause boiling in impulse lines resulting in vapours continuously getting replaced by fresh liquid from the tank and creating circulation with pulsating flow. This shall come in the way of measurements.

3. The Proposed Simplified Solution:

o By positioning the transmitter at the top of the tank, you avoid direct exposure to low-temperature liquids.

o Connecting the tank's top to the low-pressure side and the bottom to the high-pressure side of the transmitter. Provide no insulation on impulse lines. This can mitigate the risks associated with low-temperature liquids affecting the transmitters.

o Any low-temperature liquid entering the high-pressure side will vaporize immediately, which prevents the liquid from compromising the transmitter and ensures that only vapor is present in both impulse lines.

4. Advantages of the Proposed Solution:

o The impulse lines become self purging.

o Simplified calibration and maintenance due to more accessible positioning of the transmitter.

o No risk of transmitter malfunction due to low-temperature exposure.

o No heat loss in impulse lines in view of that getting filled by thermally non- conducting vapours at ambient temperatures.

Your idea to leverage the inherent properties of a volatile fluid and the concept of vapor pressure to simplify level measurement is clever. By ensuring only vapors occupy the impulse lines, you are using the volatility of the liquid in your favor, thereby potentially eliminating the problems you've identified with liquid presence in the impulse lines.

This is a successfully tried out solution for tanks storing liquid ethylene at - 120°C and ammonia tank at - 36°C.