Sizing Pipes and Control Valves Concurrently

Process Outline

1. Find the Approximate Control Valve Size

   1. Model the Control Valve with the appropriate settings. AFT Arrow will determine the required loss.

   2. Add the Control Valve to an Engineering Library.

      1. Attach a Cost Library to the Engineering Library if one does not already exist.

      2. Create a C_v vs. Cost Scale Table for the control valve with estimated cost values.

   3. Select the appropriate options in the Sizing Window.

      1. The cost for the control valve will only be included in the Objective if it is specified to be included in Size/Cost Assignments.

      2. If there are several control valves that will ultimately be of the same type, they should be placed into a Maximum Cost Group.

   4. Run ANS.

      1. Investigate the resulting control valve size.

      2. Update the cost information if necessary. If the control valve accounts for a large portion of the overall cost, accurate values here are more important.

      3. Repeat the process as necessary, until reasonable estimates for cost at the resulting operating point are obtained.

2. Compare Candidate Control Valves

   1. Locate accurate loss and cost information for the candidates.

      1. Incorporate these values into Engineering and Cost Libraries.

   2. Compare the sizing results for each candidate control valve.

Reference Pressure Concerns

It is impossible to generate a unique solution from a system (or part of a system) entirely bounded by fixed flows. For an in-depth discussion of this issue and common areas where it may occur see Role of Pressure Junctions - Detailed Discussion.

Particularly common when sizing a system is the tendency to place a Flow Control Valve immediately downstream of a Compressor modeled as a fixed flow. This situation has no unique solution, as discussed in the Sizing Compressors or Fans with Flow Control Valves section of the previous topic.

The typical solution to this issue is to temporarily change the Flow Control Valves to Pressure Drop Control Valves where necessary, until a compressor curve is selected and specified. Effectively, this is the equivalent of specifying that our Flow Control Valve has a pressure drop requirement equal to the Pressure Drop Control Valve setpoint.

Control Valves and Design Requirements

ANS will always attempt to minimize or maximize the value of the Objective. ANS does not know if the resulting design is realistic and this has no impact on the sizing. It can present results that would be physically impossible to attain in the field.

AFT Arrow is no different in this regard - it is possible to specify a system that would require physically unrealistic behavior. For example, with an AFT Standard fluid, the gas temperatures are allowed to drop below saturation temperature. The system will still be solved as though the fluid is completely superheated gas. When possible, there will be Warnings if this type of behavior is detected, but the solver does not make any attempt to prevent this situation. To avoid these unrealistic configurations, the user must inspect the output and determine the changes required on the system to avoid the situation. In other words, the user must adjust the design of the system to meet desired or realistic behavior - a Design Requirement.

As an analysis tool, AFT Arrow does not allow the user to directly impose these requirements. ANS, on the other hand, is a design tool and actively selects inputs to meet certain specifications. It would not be unusual to specify a minimum temperature Design Requirement to prevent the saturation issue.

How does this relate to Control Valves? Control Valves in AFT Arrow will, by default, force control. This means that the system will be able to behave unrealistically in order to meet the setpoint specified, which can be useful in a design stage. As it would not be useful for a Control Valve to be unable to achieve its setpoint during the sizing process, Always Control (Never Fail) is automatically enabled for sizing.

Consider a simple case of a Flow Control Valve regulating flow between two reservoirs. It is desired to size this system for lowest pipe weight.

Because of Always Control (Never Fail), the Flow Control Valve is essentially able to act like a compressor - providing "free" pressure to the system in order to meet the setpoint. As the cost of this "compressor" is not considered, the sizing process is trivial - we can simply select the smallest pipe size available, as our "compressor" will simply make up for the increase in pressure loss.

Clearly, it is not realistic for a Control Valve to add pressure to a system. So why don't we allow our Flow Control Valve to not meet its setpoint, as it would in reality? If the Control Valve is allowed to do so, we are essentially stating that the flow it is attempting to maintain is not actually a requirement. If this is the case, then again we will find that the "ideal" sizing solution is to select the smallest pipe size available. The Flow Control Valve will fully open in an attempt to allow more flow, but in the end the flow will be below the setpoint. However, our pipe weight will certainly be minimized.

What is the solution? We need to add a Design Requirement. It may seem intuitive to add a minimum flow requirement to our system as we are attempting to control flow. However, this will not solve the problem because the Control Valve will still simply add pressure to meet the setpoint. Furthermore, adding Design Requirements that conflict with the model itself can cause problems.

Instead, a Design Requirement should be added related to the position of the Control Valve. The position can be represented either directly as an Open Percentage if that data is available, or as a form of loss. It is typical that a Control Valve is designed to operated within a certain range of positions - often 20-80% open. It would not be useful to have a Control Valve designed to operate at 100% open, as a slight variation in flow could cause it to not meet its setpoint and open fully.

By adding a Design Requirement on minimum dP/dH or maximum C_v/Open Percentage we prevent unrealistic behavior and return a meaningful sizing result.

Note: It is always required to add this Design Requirement for Control Valves in the sizing model - even if the junction is not marked to be Sized in Size/Cost Assignments. Including a Control Valve in Sizing adds its cost to the Objective which can influence the result, which is not always a concern. However, the behavior described above is always true and hence always needs a Design Requirement.

Optimal Control System Operating Point

Just like sizing pipes and compressors concurrently, there is an optimal size of a control valve that minimizes total system cost. Approaching the problem in "stages" rather than concurrently can easily miss the lowest overall cost solution.

Like the compressor process, we do not know specifically what Control Valve is going to be used in the early stages of the process so we need to estimate the cost.

For Control Valves, the logical choice is to estimate with a Cost vs C_v relationship. This information will be represented for the particular component in a Scale Table.

Note: When defining a Cost vs C_v Scale Table, keep in mind that this table is relating the current C_v to the cost of the device. For example, if the Control Valve has a Design Requirement to be 80% open, and ANS calculates a corresponding C_v of 120, the cost used from the Scale Table will be that for a C_v of 120, not the full-open value of the control valve. This is important to keep in mind when creating the scale table, as manufacturer cost information will often be in the form of full-open C_v vs cost.

The Approximate Sizing Solution

When the initial model is defined, the sizing configuration complete, and the control valves appropriately defined with approximate cost information, the automated sizing can take place.

ANS will size the pipes of the system to minimize cost while obeying Design Requirements - including the Design Requirement on position discussed above. However, this is still an estimate because we are not modeling a real control valve.

If the control valve costs are a significant portion of the overall cost, the estimates made for cost should be refined and the system re-sized before proceeding.

Sizing with a Real Control Valve

Now that there is a good idea of the size and cost of control valve we are looking for, we can select a real control valve we may want to use.

It will rarely be clear which control valve is the best choice. First, the exact operating point determined from the approximate solution is likely unavailable commercially. Second, different valves have different costs - even if a valve with the exactly correct specifications is available, it may be much more expensive than one that is slightly mismatched. This cost difference could mean the cheaper valve is better overall, despite not appearing to be the ideal choice.

The best course is to select several candidate valves, and size the system independently with each candidate. It is important that accurate loss and cost information be included for each candidate, as this no longer meant to be an estimation. After testing each candidate, the system with the lowest overall cost can be selected.

If the sizing results differ significantly in this stage this could indicate that the control valve is poorly matched, that there was an error in the process, or that the system is dominated by control valve costs and therefore very sensitive to the estimations.