Seal Specifications, How to Write Them
It is extremely important for any modern process company to have a good set of seal specifications. When written properly the specifications will:
Classify seals by operating conditions:
- Some seal suppliers recommend the use of rotating seals (the spring rotates with the shaft) to a maximum of 5000 feet per minute (25 meters per sec.) as measured at the seal faces, and stationary seals for speeds greater than 5000 feet per minute (25 meters per second) as measured at the seal faces. I believe you are always better off with stationary sealing designs if you can justify the higher inital cost.
- Specify balanced seals for stuffing box pressures of one Torr vacuum to 400 psi. (28 bar) Note: this is not discharge pressure. The pressure in the pump stuffing box is often very close to the pump's suction pressure.
- Use heavy-duty designs for greater pressures. These designs incorporate:
- Back up rings to prevent elastomer extrusion.
- Thicker cross section components were used in the past to prevent face distortion. Thinner cross sections have become popular since the introduction of finite element analysis programs.
- Lower the hydraulic load at the seal faces.
- A reduced hydraulic balance ratio. Especially with fluids having a specific gravity below 0.4.
- Specify two-way balance for dual seal applications. This will prevent opening the inner seal if barrier fluid pressure is lost, or if there is a significant increase in stuffing box pressure.
- Use O-rings to published temperature limits. If you go higher, compression set can occur.
- Use non-elastomer (metal bellows) seals for temperatures outside these limits. Petroleum products being the exception. Because of the possibility of coking in these applications, seal chamber cooling is necessary.
- Specify excessive motion designs for mixers, agitators, sleeve bearing equipment, etc. These designs incorporate:
- Wider hard faces so that the thinner carbon/ graphite face has more radial movement
- More internal clearance in the seal components.
- Axial movement capability with out compressing or extending the spring or springs and affecting proper face loading.
Use dual seals
- Any time you pump:
- Dangerous products
- Costly products
- If down time is very expensive.
- Fluids that give off fugitive emissions.
- Dual seals can be used in the following configurations:
- Rotating face to face
- Rotating tandem
- Rotating concentric
- Stationary back to back
- Stationary concentric
- Stationary tandem
- Do not specify dual seals in the following configurations
- Rotating back to back
- Stationary face to face
- Stationary tandem, if the first stationary face is positioned in the bottom of the stuffing box.
In each of these configurations the sealed product is at the inside diameter of the seal faces and sliding components. Solids in the fluid will cause problems as centrifugal force throws the solids into the seal faces and restricts the free movement of the sliding and flexing components.
The inner seal should be pressure balanced in both directions to prevent opening of the inner seal if barrier fluid (or system) pressure fluctuates or is lost. Two way balance can easily compensate for vacuum or reversing pressure.
Clearly identify all of the seal materials, otherwise it will be impossible to troubleshoot a premature seal failure.
- Carbon/graphite. Only unfilled grades are acceptable with the exception of cryogenic service where a special self-lubricating grade is necessary. Do not use a carbon /graphite if you are sealing petroleum products and you have to meet fugitive emission standards. The small pits that form in the face will not allow the seal to pass a fugitive emission test.
- Hard faces. Alpha sintered silicon carbide, and nickel base tungsten carbide are satisfactory faces for most applications
- Elastomers. Use the O-ring configuration. Any of the acceptable and available compounds are satisfactory. The O-rings must not be spring loaded or glued in any manner. When using Viton® specify only those grades that can be used with water and steam.
- Metal components. Any corrosion resistant material compatible with the pumped fluid and pump components is acceptable. The springs or metal bellows should be manufactured form hastelloy "C" or any other acceptable material not subject to chloride stress corrosion.
Desirable features in any mechanical seal.
- Centering ability. You want to start out with the narrow softer face (carbon/graphite) in the center of the wider hard face so that the seal can compensate for maximum shaft deflection. Most of the centering can be accomplished by:
- Piloting the gland to the inside of the stuffing box.
- Piloting the gland to the outside diameter of the stuffing box face.
- Placing shim stock at the inside diameter of the stationary face.
- Use cartridge seal centering clips.
- Specify anti-clogging features:
- Springs out of the fluid.
- The dynamic elastomer must move to a clean location.
- Keep the sealing fluid at the seal inside diameter so that centrifugal force throws the solid particles away from the lapped seal faces and flexible components.
- Non-fretting designs with no dynamic elastomers touching the shaft or sleeve. Solid shafts should be specified to help resist shaft deflection at start up and when operating off of the pump curve's best efficiency point.
- Built in environmental controls where possible.
- Flush/ recirculation/ vent connection.
- A vent and drain/ quench connection.
- Disaster bushing for bearing failure and personnel protection.
- The smallest cross-section possible to maximize the room in the narrow stuffing box. You need this room for radial shaft movement and for some place to centrifuge heavier solids in the liquid. A better design would place the entire seal in a gland positioned between the stuffing box and the pump bearing case where there is plenty of room for a good clearance at the seal face outside diameter.
- Cartridge designs for open impeller adjustment and ease of assembly. Be sure the cartridge is sealed to the shaft or sleeve at the inboard end. Outboard sealing can let solids penetrate between the seal sleeve and the shaft making the seal removal difficult
- Split designs for easiest installation and to avoid re-alignment between the equipment and its driver.
- Glued elastomers are never acceptable. The glue will cause a hard spot to form in the elastomer causing the dynamic elastomer to leak.
- Vibration damping, especially with metal bellows designs.
- Slotted or other forms of non-dedicated glands for maximum flexibility.
- Rotate the fluid in the stuffing box rather than have the seal components rotate through the fluid. This is especially important in metal bellows designs because of their thin plates. Abrasive fluids can cause severe wear in a short period of time.
Present and more restrictive "right to know" legislation will discourage the shipment of seals to outside repair facilities or back to the manufacturer because of personnel hazard problems. Try to select designs that can easily be cleaned and the critical parts replaced at your facility. The following components must always be replaced:
- The carbon-graphite face. Relapping is not acceptable because trapped solids and lapping powder will be forced into the soft carbon. Machining or grinding a blank carbon is not acceptable because the blank will not have enough density when the outer layers are removed.
- The springs work harden with time and corrode at a faster rate than the other metal components.
- All of the elastomers (rubber parts). This included Teflon® wedges, chevrons, O-rings, etc.
- Set screws. They are not normally manufactured from hardened material because of corrosion resistance, and if used several times will "mushroom" and lose their holding ability.
- Relapping of the hard face is acceptable as long as there are no cracks or chips visible.
- On February 17, 2018