White Paper: Sizing Differential Pressure Regulators …

1 White Paper D352289X012. September 2015 Differential Pressure Regulators Sizing Differential Pressure Regulators in Seal Oil Service for Turbomachinery Selecting the proper Differential Pressure regulator Differential control Regulators operate in much the can sometimes be perceived as a confusing prospect. same way as a standard regulator, however they Differential Pressure regulator applications usually have an additional loading Pressure added to the have operating conditions outside of what is normally top of the regulator diaphragm, which increases the seen in standard Pressure regulating operations, which force and raises the set point of the unit. Figure 2. means more variables must be taken into account details the loading Pressure applied to the Differential when Sizing and selecting a Pressure control device. control regulator for both Pressure reducing and backpressure service. Differences between Standard and Differential Control Standard, self-operated Regulators are self contained Pressure REDUCING BACKPRESSURE.

2 Valve and actuator combinations that rely on three essential elements to control Pressure either downstream ( Pressure reducing) or upstream (backpressure) of the valve. The three elements a regulator uses are: a valve plug and seat to control flow, a diaphragm to measure system Pressure and a spring to load the diaphragm ADDITIONAL LOADING. and provide a force balance to create a regulator set Pressure ADDED TO. INCREASE SETPOINT. point. Figure 1 shows a diagram depicting the three essential elements in both a Pressure reducing and backpressure regulator. M1233 M1241. Pressure REDUCING BACKPRESSURE. INLET. INLET Pressure . Pressure . OUTLET. OUTLET Pressure . Pressure . INLET Pressure . SPRING PROVIDES. INLET Pressure ATMOSPHERIC. OUTLET. ATMOSPHERIC Pressure . Pressure . Pressure . ATMOSPHERIC Pressure . OUTLET Pressure . ATMOSPHERIC Pressure LOADING Pressure . EQUILIBRIUM AND. SETPOINT. Figure 2. Elements of a Differential Pressure DIAPHRAGM SENSES Controlling Regulator Pressure CHANGES.

3 M1231 M1239. PLUG AND SEAT CONTROL FLOW. THROUGH THE VALVE BODY. NLET Pressure INLET Pressure . OUTLET Pressure . ATMOSPHERIC Pressure . LOADING Pressure . INLET. INLET Pressure . Pressure OUTLET Pressure . ATMOSPHERIC Pressure . OUTLET. OUTLET Pressure . Pressure . LOADING Pressure . ATMOSPHERIC. ATMOSPHERIC Pressure . Pressure . LOADING Pressure . Figure 1. Essential Elements of a Standard Pressure Controlling Regulator White Paper D352289X012. September 2015 Page 2 Differential Pressure Regulators Seal Oil in Turbomachinery FORWARD Pressure CONTROL REGULATOR. One application where it is fairly common to see The more common control method is forward Pressure control Differential Pressure regulation is in seal oil systems for which uses a Differential Pressure regulator to maintain the seal oil supply Pressure . The reference gas Pressure is normally large rotating equipment like compressors or pumps. supplied from the process side of the seal. Typical Differential In these systems pressurized oil is used in the seals to Pressure range is 25 to 50 psi.

4 Keep process fluid from leaking out of the machine via the shaft. The benefit of using a Differential Pressure regulator in this application is that the regulator is always sensing the reference Pressure coming from the inner seal, which means the regulator can make immediate flow corrections when it senses a Pressure change in the reference fluid Pressure . In the case of seals, the purpose of the regulator is to REFERENCE. provide a supply of oil at a Pressure slightly higher than that of the process fluid inside of the difference in Pressure is maintained by the reference Pressure , which comes from the inner seal, and the spring force provided by the regulator setpoint. FILTER. There are two common methods for controlling this Differential Pressure : forward Pressure control or backpressure control. Both of these methods are viable at accurately controlling seal oil Pressure , but the COOLER. forward Pressure control method is more prevalent. Forward Pressure Control Forward Pressure control uses a Pressure -loaded Pressure reducing regulator that will use the Pressure of the reference gas to increase the set point of the regulator.

5 An example seal oil system with forward Pressure control is detailed in Figure 3. RESERVOIR. Before start-up of the pump or compressor, the seal oil system is started to allow oil to enter the seals. SPECIAL PURPOSE OIL SYSTEM. Because there is no loading Pressure being added (API 655-PART2). to the regulator, the Pressure reducing regulator is controlling downstream Pressure to the initial regulator setpoint. Figure 4 shows the process Figure 3. Example of Forward Pressure Control Using a Differential Regulator conditions across an example regulator during oil pump startup. Once the seal oil system is running, the rotating equipment will be started and the Pressure in the regulator spring case will begin to increase. As a result the regulator set point will begin to rise and the new setpoint of the regulator will become the spring set point plus the reference gas Pressure . White Paper D352289X012. September 2015 Page 3 Differential Pressure Regulators SEAL BACKPRESSURE REGULATOR.

6 Backpressure control uses a Differential Pressure regulator to maintain the required seal Pressure and flow. The reference is placed on the sweet oil drain back to the reservoir. SETPOINT = SPRING SETTING + LOADING Pressure . REFERENCE. FILTER. Backpressure Regulator Depending on the system design, backpressure Regulators are not always required. M1231. COOLER. INLET. INLET Pressure . Pressure - PROVIDED BY PUMP. OUTLET. OUTLET Pressure . Pressure - CONTROLLED BY REGULATOR. ATMOSPHERIC. ATMOSPHERIC PRESSUREPRESSURE. - 0 psig AT PUMP STARTUP. LOADING Pressure . Figure 4. Pressure Reducing Regulator at Startup Backpressure Control Backpressure control is the second method that can RESERVOIR. be used to control Differential Pressure . An example of backpressure control is shown in Figure 5. In SPECIAL PURPOSE OIL SYSTEM. (API 655-PART2). backpressure control the regulator is placed on the downstream side of the seals, sensing Pressure upstream of the regulator coming from the seals. As Figure 5.

7 Example of Backpressure Control Using a the Pressure through the seals begins to increase to Differential Regulator the regulator set point, the backpressure regulator will start to open and allow excess Pressure to relieve downstream, keeping the Pressure through the seals constant. Similar to forward Pressure control, this control method will also utilize Differential control using the reference Pressure provided from the inner seal. White Paper D352289X012. September 2015 Page 4 Differential Pressure Regulators Pressure Loaded Regulator Mechanics This is because even though the inlet pressures and Sizing a Differential regulator for seal oil service can setpoints are different, the Pressure Differential be challenging since the manufacturer's literature between the inlet Pressure and the loading Pressure frequently does not include set points as high as is the same. Both Regulators in Figure 6 are open the the combined spring load plus loading Pressure . For same amount and therefore have the same Cv value.

8 Example, the capacity tables for the Type MR95HD Therefore, the Cv tables in the product bulletin can be only have data for Pressure settings up to 150 psig. used to size a Differential regulator the same way you Fortunately, the addition of loading Pressure has no size a standard Pressure reducing application. effect on the regulator flow coefficient, otherwise The same principle applies when Sizing a backpressure known as its regulating Cv value. The regulating Cv is regulator. Figure 7 details pre- and post-compressor characterized by the regulator body and the initial startup conditions for a backpressure regulator and spring setting of the regulator, and the addition of shows that fundamentally the regulator still operates loading Pressure provides a force balance across in the same manner detailed above. the regulator. This keeps the regulating Cv value SPRING SETTING = 20 psig SPRING SETTING = 20 psig dependent upon the initial spring setting. LOADING Pressure = 0 psig LOADING Pressure = 400 psig SPRING SETTING = 20 psig SPRING SETTING = 20 psig LOADING Pressure = 0 psig LOADING Pressure = 400 psig P1 = 20 psig P1 = 420 psig P1 = 500 psig P2 = 20 psig P1 = 500 psig P2 = 420 psig M1240.

9 M1233. INLET. INLET Pressure . Pressure . INLET. INLET Pressure . Pressure OUTLET. OUTLET Pressure . Pressure . OUTLET. OUTLET Pressure . Pressure ATMOSPHERIC. ATMOSPHERIC Pressure . Pressure . ATMOSPHERIC. ATMOSPHERIC Pressure . Pressure LOADING. LOADING Pressure . Pressure . INLET Pressure . E. URE LOADING. LOADING Pressure . Pressure . OUTLET Pressure . RESSURE LOADING Pressure . URE Figure 7. Pressure Conditions in Differential Backpressure Figure 6. Pressure Conditions in Differential Pressure Regulators Pre- and Post-Compressor Startup Regulators Pre- and Post-Compressor Startup Consider the two Regulators shown in Figure 6. The One key difference in backpressure operation is that as regulator on the left uses spring load only to make a the loading Pressure increases, the Differential Pressure setpoint of 20 psig. The regulator on the right uses across the valve itself also increases. This is inverse to the same spring load plus a Pressure load of 400 psi to the Pressure reducing example from Figure 6.

10 In liquid make a set point to 420 psig. At 4 psid droop (or 20% service, maximum flow is proportional to Pressure droop off spring setting), the regulator on the right Differential . In forward Pressure reducing Regulators , as flows much less than the regulator on the left since the loading Pressure increases, maximum flow decreases. Differential Pressure across the valve is considerably In backpressure Regulators , maximum flow increases less. However, when the Cv coefficients are calculated when loading Pressure increases. for each regulator, the values are the same. White Paper D352289X012. September 2015 Page 5 Differential Pressure Regulators Sizing a Seal Oil Regulator for Forward Table 2. Required Cv Calculations Pressure Control PRE-COMPRESSOR STARTUP POST-COMPRESSOR STARTUP. Below is a step-by-step guide for Sizing a Differential 80 GPM 80 GPM. Pressure regulator using a Type MR95 HDP in seal Cv =. 480 psi Cv =. 80 psi oil service. We will be using the Pressure conditions from Figure 6 as the basis for our Sizing example.