Use of Multi-step Control Valves

1. Purpose

This procedure identifies criteria for the use of multi-step control valves, and the P&ID representation.

2. General

Engineers preparing Instrument Schedule “A” Project Specifications (600 series specifications) shall use this procedure to ensure that a multi-step control valve is used where required and that the depiction on the P&ID is correct.

3. Selection Table

** Now up to 600°F (316°C) with metal seat (Class V). With soft seat (which we generally would not use), the temperature limit is 450°F (232°C). For hydroprocessing units; the Hot Separator will continue to use the temperature limit of 450°F (232°C) when deciding to use the 77000 series vs. the 78400 series.

4. P&ID Depiction for FLASHING Services

Note: For locations where only an HV is needed in flashing service (such as a manual drain at a compressor suction drum) delete the actuator and label the valve as an HV. Other aspects would be the same as the picture below.

5. P&ID Depiction for NON-flashing Services

6. Background

For valves in high differential pressure applications with bubble point liquid, a straight run of pipe and a series of reducers are used to reduce pressure loss at the inlet to minimize flashing upstream of the valve.

As a result of experience with erosion and vibration on the outlet piping on the earliest high pressure units, a straight run of pipe and a series of expansions in size are used at the outlet.

The multiple reducers / expanders at the valve inlet and outlet are intended to gradually accelerate and decelerate the fluid upstream and downstream of the valve in order to avoid an unnecessarily large step change in velocity at a single reducer or expander.

The straight runs between reducers / expanders / fittings are intended to allow the fluid to approach a final average velocity in a line segment before encountering the next reduction or expansion or elbow or tee.

The straight runs are intended to make all the accelerations and decelerations occur in a straight line path rather than in elbows, tees or other fittings.

Prior to the use of special multi-stage valves and prior to the use of associated special piping arrangements, vibration problems were encountered at control valves in high pressure drop service. The solution to the problems was the simultaneous application of both the special multi-stage angle valves and the associated special piping arrangement.

It is difficult to say whether the multi-stage valve alone, without special piping arrangements, is adequate for eliminating potential vibration problems. Inflection Point Engineering prefers to be conservative in high pressure drop service, particularly in flashing or degassing service, and to require the special piping arrangement.

7. Application Notes

1. Where Inflection Point Engineering would normally show an “E” assembly, the bypass valve will also need to be a multi-step valve with a hand actuator (such as HP boiler feedwater to a desuperheater, or HP steam condensate to LP Condensate header) and detailed out.

The bypass valve on the P&ID looks just like a gate valve. An actuator is not shown on the P&ID. This valve is tagged as an HV. One block valve is put on the downstream side.

2. For the 78400/18400 series valves, review the ‘Staging Ratios & Pressure Drop Guidelines” in the brochure to ensure the trim type (A, B, or C) can handle the dP. Specify a Cv from the list of available Cvs for the number of stages / trim type required to suit the valve pressure drop and service (intermittent service or continuous service). It is OK to use more stages than necessary in order to obtain the desired Cv

3. There are additional "small" Cv's that are available in the 78400/18400 series valves but not listed in the vendor literature. Note: ~ 800 psi dP / stage used by Masoneilan.

For the Standard Capacity – 4 Stage Design Cv = 0.4 Cv = 0.6

For the Standard Capacity – 6 Stage Design Cv = 0.04 Cv = 0.08 Cv = 0.2 Cv = 0.3 Cv = 0.5

4. For all the multi-step valves (77000 and 78400/18400 series), check the valve inlet velocity to make sure it is less than 40 ft/s,. (It may be necessary to do a liquid only Cv calculation in the 4D 616 spec calculation section to estimate the valve inlet velocity). To calculate the valve inlet velocity, use either the 4D 616 spec Calculations section or tool “” (at Documentation System \ Tools \ Technology & Skill Center Specific \ Hydroprocessing). It is sometimes necessary to use a larger body size with reduced trim to limit the valve inlet velocity to Masoneilan’s recommended maximum of 40 ft/sec for multi-stage valves. This situation tends to occur at hot separator level control valves because the temperature is high, the liquid is less dense, and that contributes to higher velocities. For these valves, always select a valve body size large enough to meet this 40 ft/sec inlet velocity limit in the “” flow case, based on a valve inlet port diameter equivalent to Schedule 160 XXS pipe.

5. For the 77000 series valve, the valve Cv that is shown in the catalog for a given body size, is also available in the next larger body size. If necessary to meet the valve inlet velocity limits, specify the Cv that is required, but in a valve body size one size larger than listed in the Masoneilan 77000 series valve catalog. Note: ~ 500 psi dP / stage used by Masoneilan. Trim A is 7 – 9 stages. Trim B is 5 stages. Trim C is only 1 stage and should not be used. Other 77000 series body size and Cv selections available are:

1 x 1½ body Cv = 4

1 x 2 body Cv = 1, 2, and 4

1½ x 2 body Cv = 7

2 x 3 body Cv = 6

2 x 3 body Cv = 9

8” x 8” or 8” x 10” body, Cv = 185 (and reduced trim Cv = 125 and 85).

6. For angle valves (77000 and 78400 only), there should be a note on the P&ID that states: “Orient inlet and outlet piping in horizontal plane”

7. Fisher valves are also listed in the specs.

For: Masoneilan 77000 ≈ Fisher DST-G (these are angle valves) Masoneilan 78400 ≈ Fisher DST (these are angle valves) Masoneilan 18400 ≈ Fisher DST (these are GLOBE valves)

8. The swaging criteria, after determining the required valve Cv and size, are:

Inlet Starting from the valve inlet and going upstream: The inlet piping should be a straight run of 5 pipe diameters, the same size as the valve inlet. The upstream block valves are in one line size larger section of pipe. The reducers upstream of the block valves, if required, are a maximum of two line sizes.

The inlet line from the valve inlet to just upstream of the most upstream reducer should be a straight run.

Outlet: Take one line size expansion at the valve. The downstream block valve is in this one line size larger section of pipe. Expanders on the outlet after the block valve, if required, are spaced at 5 pipe diameters minimum to reduce potential for vibration. The first expander after the block valve is two line sizes and the remaining expanders are a maximum of two line sizes.

Use 5 pipe diameters after the last expander to any tee or ell. The outlet line from the valve outlet should be a straight run through the last expander and preferably through 5 pipe diameters after it.

These swaging criteria are generally based on Standard Specification 8-11 for piping. The Design Engineer should provide process and hydraulic information to size these valves before the internal P&ID check is completed so that the correct number of reductions and expansions can be shown on Revision 0 of the P&ID. If this is not possible, the corrections will need to be made on a later revision.

Usually there is no advantage to using a larger body size with reduced trim size unless the valve inlet velocity dictates a larger body size (Section 7.4). A larger valve body allows fewer pipe size reductions but it, along with its larger block valves, increases the capital costs.