AMACS Design Philosophy – Gas/Liquid Separations Part I
To achieve high-efficiency gas/liquid separation, one must analyze the entire vessel from inlet piping to outlet nozzle. Most designers only focus on the mist eliminator and overlook the other sections of the column. AMACS’ design philosophy is to evaluate each of the five sections in a vessel collectively, which are applicable to vertical and horizontal separators and consist of:
- Inlet Piping
- Inlet Nozzle/Device
- Liquid Surface
- Gravity Section
- Mist Elimination Section
Section I of AMACS design philosophy for gas/liquid separations will concentrate on vessel inlet piping. Inlet piping plays an important role in gas/liquid separation and can affect the performance of a vessel regardless of the internals installed inside the column. AMACS design philosophy for inlet piping is geared towards critical applications where elite liquid separation is required, there is a high liquid loading, and/or the inlet momentum is excessive. In other cases, some exceptions can be made to this philosophy which will be discussed below. This philosophy is common among the separations industry and are well documented in several design practices.
Ideal Inlet Piping
The most recommended inlet piping design is to have the pipe travel straight into the vessel, parallel to the floor. The nearest disturbance (valve, reducer, or bend in piping) should be at least 10 times the pipe diameter distance away from the vessel. This design decreases the fluid maldistribution leading up to the vessel.
Figure 1. Ideal straight inlet piping
Acceptable Inlet Piping
Inlet piping is considered acceptable when there is a bend in the vertical plane and the direction is downward. A bend occurring within a distance of 10 times the pipe diameter in the vertical plane and going downwards will not affect flow distribution inside the column. In this case the piping layout will be acceptable, however, not ideal.
Figure 2. Vertical bend in inlet piping
Not Recommended Inlet Piping
Inlet piping configurations are not recommended when there is a bend that occurs within 10 times the piping diameter and:
- The bend is in the horizontal plane.
- The bend is in the vertical plane and the direction is upwards.
A bend in the horizontal plane shifts all of the incoming flow to enter the vessel unevenly as the gas and liquid will likely be thrown to one side. As shown in Figure 3, this bend will cause vapor maldistribution in the vessel. This may affect scrubber efficiency due to improper utilization of the gravity section and subsequently maldistribution at the demisting device.
Figure 3. Inlet piping horizontal bend (left) and effect on flow distribution (right)
In addition, a bend in the vertical plane going upwards may allow pockets of liquid to collect and cause slug flow at the vessel inlet. Unexpected slug flow can affect the mechanical integrity of the inlet device and potentially generate more mist.
Figure 4. Upward Bend in inlet piping close to the vessel
Exceptions to these options can be applied when critical gas/liquid separation is not required. When there is a small amount of liquid and the piping is large enough to create a low inlet momentum then these piping formations can be utilized if necessary.
For critical applications where these bends are unavoidable, certain steps need to be taken to minimize the effects from the inlet piping arrangement. Computational Fluid Dynamics (CFD) is an invaluable tool to provide design verifications for new and existing configurations. CFD analysis will not only help visualize maldistribution and the impact of the bend but also provides detailed quantitative insight such as velocities, droplet behaviors, and turbulent dissipation rates. One can employ other internals to assist with gas distribution and maximize the separation efficiency inside the column.