API Piping Plan 62: A Reliable Quench System

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1 MAY05PUMPS&SYSp /19/05 4:04 PM Page 24 It s one of the few methods you have to control the environment outside a mechanical seal. By Keith Schindler, PE, Schindler Engineering, Inc., and Paul McMahan, Flowserve Corporation API Piping Plan 62: A Reliable Quench System For years, users and manufacturers have attempted to increase mechanical seal reliability. Focus often has been placed on the seal flush arrangement and methods to control the parameters that affect the seal environment. Engineering improvements have been made in seal design, face material combinations and configurations to comply with EPA s low-emissions regulations and the American Petroleum Institute s (API) recommendations. However, little attention has been given to the environment outside the mechanical seal (atmospheric side) and how this environment affects seal reliability. This article discusses one of the few methods used to control the environment external to the seal. An appendix beginning on page 28 outlines guidelines for operation, shutdown and troubleshooting of these systems. 24

2 MAY05PUMPS&SYSp /17/05 3:46 PM Page 25 An external fluid quench system (API Piping Plan 62) is designed to immerse the outside of a mechanical seal assembly and the secondary seals with a clean nonhazardous fluid [1, 2]. This fluid serves multiple purposes. 1) It occludes the atmosphere from the atmospheric side of the mechanical seal assembly, thereby reducing oxidation. 2) Since it is at a slightly higher pressure than atmospheric, it provides a low flow through the area to assist in drainage or removal of product leakage across the seal faces. 3) Depending on temperature and solubility of the pumpage in the quench fluid, it helps to prevent solidification product leakage until such leakage has exited the close clearances encountered in the gland. This build-up of product, if not removed, will otherwise accumulate to cause the seal assembly to hang up. 4) For hydrocarbon products at or near their auto-ignition point, the quench fluid cools the normal leakage from the seal assembly to a temperature below the product autoignition temperature. When considering an external fluid quench system, certain requirements must be incorporated in the design of the seal for the quench system to be reliable. The mechanical seal gland must be equipped with inlet and outlet (drain) quench connections. The quench fluid must drain through the outlet connection, making it necessary for the outlet connection to be lower than the inlet connection. This arrangement will help wash away any product leakage at the seal faces. The drain connection should be located at the lowest possible point on the seal gland to prevent a dead spot where product leakage can accumulate and affect seal performance. The mechanical seal gland must be equipped with a close-clearance throttle bushing or other suitable device. Since the quench fluid is fed to the gland under pressure, the pressure forces the fluid out of the gland toward the driver or bearings through the space between the shaft and gland ring bore. The closeclearance bushing di-rects the quench flow and any product leakage to the drain instead of along the shaft, and reduces the amount of quench fluid leakage from entering the bearing housing and contaminating the bearing lubrication. The bushing contains the quench fluid in the area of the seal assembly where it is needed, by directing the quench fluid to the atmospheric side of the seal assembly and the secondary seals. The bushing also acts as an orifice to control the amount of leakage to the atmosphere in the event of a catastrophic seal failure. Applications In applications such as hot hydrocarbons and caustics, where oxidation, coking or crystal formation (crystallization) is a problem on the atmospheric side of the mechanical seal, steam is typically used as the quench fluid. The steam is introduced into the inlet quench connection where it is directed around the sealing faces. The steam quench acts as a coolant for the mechanical seal in high temperature applications (>300 F) [2]. The steam cools the leakage on the atmospheric side of the sealing faces, which reduces coke formation and subsequent seal hang-up. For hydrocarbon product streams at or near their auto-ignition point, the steam helps cool the normal leakage from the seal assembly below its auto-ignition temperature. It also reduces cooling of the seal faces and the product in viscous or high freezing-pointtemperature applications while the pump is not running. Damage can occur to the mechanical seal at start-up, if the product thickens or freezes at the seal faces. For a steam quench application with enough space in the pump, the mechanical seal gland should be equipped with an anticoking baffle [2]. (Figure 1). The anti-coking baffle routes the steam to the area of the seal where solids collect, then directs the steam so that it carries the material away from the seal and sealing faces to the outlet (drain) connection [2]. The steam quench pressure should be regulated between 1 3 PSIG at the inlet quench connection to prevent any seal problems. Higher-than-recommended steam pressures could affect the performance of the seal and will cause the excess steam to exhaust out of the seal gland. Excess steam will cause moisture to enter the pump bearing housing if the bearing housing is not properly protected [3]. When steam is used as a quench medium for high-temperature applications, it is essential that the steam quench be operated in a gaseous state and liquid water must not be introduced into the gland area. If condensed steam is introduced to the atmospheric side of the seal assembly, it may result in flashing of the steam in the seal face area, with the consequence of unstable hydraulic forces affecting the atmospheric side of the seal faces. Another possible consequence is damage to secondary graphite sealing members either through galvanic corrosion or damage PUMPS & SYSTEMS MAY

3 MAY05PUMPS&SYSp /17/05 3:49 PM Page 26 Figure 1. Mechanical seal with anti-coking baffle that occurs when the condensation trapped in the secondary sealing member area flashes back to steam. Figure 2 shows the recommended piping system to ensure that dry steam is available as a quench fluid. If the steam supply line to the equipment is longer than 25 feet, or is taken from a high pressure source where there is more than a 50-PSI drop across the controller, reducing the initial steam pressure to a usable level for the quench, additional measures to prevent condensation in the steam quench system may be needed. Insulation and tracing of the steam supply system is one option. Others include using the heat of the equipment to reheat the steam quench by wrapping a short section of tubing around the equipment, or routing the steam quench through a water jacket on the equipment. In applications where oxidation or coking is a problem on the atmospheric side of the mechanical seal and steam is not available or can t be used, nitrogen is the second most common quench fluid used (as shown in Figure 3). Similar to the steam quench application, a nitrogen quench is introduced into the quench inlet connection of the seal gland where it is directed under the sealing faces. The nitrogen quench purges the oxygen from the atmospheric side of the seal to reduce oxidation and the flow of fluid helps remove any build-up from under the seal faces to prevent seal hang-up. The nitrogen quench also acts as a coolant for the seal in hightemperature applications by cooling the leakage on the atmospheric side of the sealing faces, which reduces or prevents coke Circle 112 on Reader Service Card 26

4 MAY05PUMPS&SYSp /17/05 3:46 PM Page 27 formation and subsequent seal hang-up in hot services (>300 F) [2]. Since nitrogen is a gas and the recommended flow rate is low, there is no visible evidence at the seal gland that the nitrogen quench system is in service [3]. The only indication that the system is working properly is a flow reading from a reliable flow meter. The nitrogen quench flow rate should be controlled to approximately one (1) standard cubic foot per minute (scfm). Higher-than-recommended nitrogen flow rates will not affect the performance of the seal, but will substantially increase the operating cost of the external quench piping system [3]. Water is a quench fluid used in sour water, caustic soda and saltwater brine applications, where the product crystallizes on the atmospheric side of the mechanical seal. The water quench fluid is introduced into the inlet quench connection of the seal gland where it is directed under the sealing faces (Figure 4). The flow of water washes away any build-up from under the sealing faces to prevent seal hang-up. The quench pressure should be limited to prevent any seal problems. The flush rate should be controlled so that a slow drip appears at the back side of the seal gland. This should be enough to keep any crystals from forming under the sealing faces to prevent seal hang-up. A water quench typically becomes a housekeeping problem. The water drips on the pump bracket and the pump baseplate, accelerating corrosion [3]. In cold climates, the water freezes and becomes a slipping hazard. For this reason, it is common practice to pipe the outlet of the drain connection on the seal gland to a suitable drain location around the equipment pad when there is minimal risk of plugging or fouling of the additional drain piping. Line-Up & Operating For an external fluid quench system to be reliable, it must be operated and maintained properly. This includes establishing clear equipment line-up, operating, preventative maintenance, shutdown and troubleshooting tasks for maintenance and operating personnel. (A detailed description of each task is listed in the attached Appendix.) Remember that the main isolation valve on an external fluid quench line should never be opened after the equipment has been prepared for service, or while it is in service. The external fluid quench system should be placed in service prior to lining up the equipment for operation and should not be isolated until the equipment is removed for maintenance. Opening the fluid quench line while the equipment is in service (HOT) can seriously damage the mechanical seal and expose operating personnel to a serious safety risk. Conclusion An external fluid quench system (API Piping Plan-62) can reliably control the environment outside a mechanical seal if the system is installed and operated properly. Designed to immerse the seal faces and the secondary seals with a clean, non-hazardous fluid, this system will remove any product leakage and build-up from the atmospheric side of the seal to prevent seal hang-up, thus extending seal life. Depending on the product or material being sealed, the quench system can be tailored to fit the application. Circle 119 on Reader Service Card PUMPS & SYSTEMS MAY

5 MAY05PUMPS&SYSp /19/05 4:01 PM Page 28 APPENDIX Equipment Line-Up & Operating Guidelines The following steps are recommended when putting an external fluid quench system (API Plan-62) into service. This line-by-line description can be used to assist maintenance and operating personnel with their daily line-up, operating and troubleshooting tasks. 1. Open main valve isolating the external quench system. 2. Follow the procedure listed below for different quench mediums. a. Steam Quench: (Fig. 2.) i. Open the drain valve to begin heating up the steam quench line and to remove the condensate from the system. ii. Once the steam quench Figure 2. API Piping Plan-62 Steam Quench Piping Layout piping is HOT and all condensate is out of the system, close drain valve. iii. Note: A small amount of condensate may drip from the seal flange. This is normal, but most of it should be removed from the quench system by the steam trap. Higher steam quench pressure will cause the excess steam to exhaust from back of seal gland and force condensate to enter bearing housing of the pump or driver. b. Nitrogen Quench: (Fig. 3.) i. Note: Throttle needle valve (or adjust pressure regulator) until flow meter reads one (1) standard cubic foot per minute (scfm). There will be no other visual indicators at the seal Circle 110 on Reader Service Card. 28

6 MAY05PUMPS&SYSp /19/05 2:32 PM Page 29 Figure 3. API Piping Plan-62 Nitrogen Quench Piping Layout gland to show nitrogen is flowing. The pressure gauge and flow meter are the only indicators to show the system is operating properly. Nitrogen is expensive. Excessive nitrogen flow rates will not damage the mechanical seal or affect its performance, but will increase operating cost of the system [3]. c. Water Quench: (See Fig. 4.) i. Open needle valve (or block valve upstream of pressure regulator) on quench line near primary seal gland to create recommended flow. ii. Throttle needle valve (or adjust pressure regulator) until a slow water drip appears at back side of seal gland. Open needle valve controlling the quench pressure (or block valve upstream of the pressure regulator) on quench line near the primary seal gland to create recommended flow. (See Table 1.) WARNING: Never open the main isolation valve on an external fluid quench line after the equipment has been prepared for service or while it is in service. The external fluid quench system should be placed in service prior to lining the equipment up for operation and should not be isolated until the equipment is removed for maintenance. Opening the fluid quench line while the equipment is in service (HOT) can seriously damage the mechanical seal and places the operating personnel at a serious safety risk. Proceed with opening all process valves to place the pump in service. Circle 120 on Reader Service Card PUMPS & SYSTEMS MAY

7 MAY05PUMPS&SYSp /19/05 2:32 PM Page 30 Figure 4. API Piping Plan-62 Water Quench Piping Layout Check for leaks around any valves and connections in the external fluid quench line. Start pump per normal start-up procedure. Once the pump is in service, check the visual indicator (exhausting steam, water dripping and pressure reading) to ensure the system is performing. Shutdown Guidelines The following steps are recommended to safely isolate an external fluid quench system for maintenance. 1. Shut down pump per normal shutdown procedure. 2. Proceed with closing all process valves to isolate pump from service. 3. Close main valve isolating the external fluid quench system. a. Steam Quench: Open drain valve to remove any condensate from system. This step is important to reduce risk of freezing during cold weather. b. Nitrogen Quench: No additional steps are required to isolate the system for maintenance. c. Water Quench: No additional steps are required to isolate the system for maintenance. 4. The equipment is now isolated and ready for maintenance. Circle 108 on Reader Service Card 30

8 MAY05PUMPS&SYSp /19/05 4:03 PM Page 31 Table 1: Recommended flow rates and/or pressures Preventative Maintenance Tasks The external fluid quench system should be checked daily during normal operator rounds. The Preventative Maintenance (PM) steps outlined below are set up to assist operating personnel in troubleshooting the system. 1. Verify that the main isolation valve for the external quench system is open. 2. Follow the next steps for the different fluid quench applications. a. Steam Quench: i. Visually note steam exhausting from back of seal gland. ii. Verify steam trap is operating properly by opening drain valve to see if there is any condensate in the system. iii. Visually note reading on pressure gauge. The external steam quench pressure should set between 1 3 PSIG. iv. Visually note reading on the flow meter (if applicable). It should read between 5 lbs/hr and 10 lbs/hr. v. Take a temperature reading at the interface of the shaft and secondary sealing device and note the temperature when the steam quench is operating properly. If the temperature varies by more than 10 F upon subsequent inspections, the steam quench system should be more thoroughly reviewed to ensure it is operating properly. b. Nitrogen Quench: i. Visually note reading on flow meter (if applicable). The flow meter should read ~ 1 scfm. ii. Visually note pressure gauge reading. It should be between 1 3 PSIG at the inlet connection. c. Water Quench: i. Visually note water dripping from back of the seal gland. ii. Visually note reading on pressure gauge (if applicable). The external water quench pressure should set between 1 3 PSIG at the inlet connection. Troubleshooting The typical failure mode for an external fluid quench system is a closed valve or clogged quench line. In the event the Preventative Maintenance indicators show that the system is not operating properly, shut down Circle 170 on Reader Service Card PUMPS & SYSTEMS MAY

9 MAY05PUMPS&SYSp /19/05 4:01 PM Page 32 the equipment according to the steps outlined previously. The tips below are set up to assist operating and maintenance personnel in troubleshooting the system. 1. Verify that main isolation valve for the external fluid quench system is open. For an external steam quench system, open drain valve to see if there is adequate pressure from the main isolation valve. 2. Open and close needle valve (or block valve) to make sure it is not plugged by debris. 3. If this proves successful, adjust pressure and put equipment back in service according to the line-up procedure. Otherwise, isolate the equipment and prepare to remove external fluid quench line for inspection. 4. Once the equipment is ready for maintenance, remove external fluid quench line from seal gland. a. Check needle valve (or pressure regulator) for pluggage. b. Check main isolation valve for pluggage. 5. If this proves successful: a. Reconnect external fluid quench piping. b. Prepare external quench system for service according to line-up procedure. 6. If the problem can t be found, contact your Mechanical Seal representative for assistance. P&S References 1. Dura Seal Manual, The Sealing Technology Guidebook, Flowserve Corporation, Copyrighted API-682 Shaft Sealing Systems for Centrifugal and Rotary Pumps. 3. API Piping Plan, Training Manual, Flowserve Corporation, Copyrighted Keith D. Schindler is a Rotating Equipment Engineer with Schindler Engineering, Inc. The firm specializes in machine repair, procedure writing, Root Cause Failure Analysis on problem equipment and operator training. Schindler holds a BS degree in Mechanical Engineering from the University of Houston, and is a Registered Professional Engineer in Michigan and Virginia. Contact him directly at SchindlerInc@aol.com Paul McMahan is the Rotating Equipment Specialist Program Manager for Flowserve FSD Seal Group in the Baton Rouge, LA, Service Center. He has more than 30 years of experience with mechanical seals and was one of the pioneers in the mechanical seal alliance concept. McMahan can be reached at pmcmahan@-flowserve.com A piping plan pocket pal with a brief description of many of the standard API piping plans, including Plan 62, is available as a download in.pdf format or as a handy pocket reference. To obtain your piping plan pocket pal, log on to com/seals/literature and request Mechanical Seal Piping Plans publication FRA