Sources of a temperature change in the stuffing box area, cheat sheet 9-8
Many factors contribute to a change in the pump stuffing box temperature :
Ambient or the surrounding conditions have a major affect on temperature:
- Is the pump located in an area of temperature extremes? Can rain or snow fall on the pump?
- Is the stuffing box area frequently washed down with a water hose?
- Is the pump insulated from the surrounding temperature?
The product its self can be the problem.
- Is the fluid a lubricant? Non lubricants can cause “slip stick” seal vibration problems.
- What is the normal pumping temperature of the product? Cold temperature can thicken fuel
- What is the conductivity of the product you are pumping? Water has good conductivity, oil has poor conductivity.
- Does the fluid have a low specific heat? The lower the specific heat the hotter it is going to get.
- What is the specific gravity of the liquid. High specific gravity consumes more motor kilowatts.
- Is the product viscous? Viscous products consume more power also. They also cause the pump to operate off the best efficiency point causing shaft deflection problems.
The seal face load is very important.
- What is the spring force? Thirty ponds per square inch (2.0 Kg/cm2 ) should be typical when the carbon face is new.
- What is the percentage of the seal face hydraulic balance? 70/30 is normal, but low specific gravity fluids (less than 0.4) should have a balance closer to 60/40.
- How wide are the faces? Wide faces generate more heat.
- The axial thrust of the shaft can increase the face load.
- The installation skill of the mechanic is critical. Does he have a print to reference or is he using the old set screw mark on the shaft?
- How high is the stuffing box pressure? Does it swing from pressure to vacuum?
- Because they are mounted outside the stuffing box thermal growth of the shaft can increase the face load in most non-metallic designs.
- The shaft speed is a major factor. The faster you go the more heat generated. Some single spring designs can unload the faces at these higher shaft speeds.
- The direction of impeller adjustment will determine if the faces will overheat after the impeller has been adjusted. Without a cartridge, the seal faces cannot be moved to compensate for the new impeller setting.
- The direction of shaft rotation can be important with some single spring designs. The wrong spring winding direction can unload the seal faces.
The face materials are another factor.
- The higher the density of the carbon/graphite mixture, the less air pockets beneath the face surface. These air pockets will hinder the heat transfer away from the face.
- Is the face insulated by an elastomer or gasket?
- Is the face installed in a metal holder? The metal holder has a much better heat conductivity than the carbon/graphite.
- The face material conductivity must be considered. Silicone carbide and tungsten carbide are the best. 85% and 99.5% ceramic are the worst with carbon/graphite falling some where in between.
- The harder the face, the less friction generated.
- The smoother the face, the less friction between the lapped faces.
- A carbon/graphite face vs a hard face will generate less heat than two hard faces running against each other.
Vacuum causes a heat problem between faces.
- If an open impeller is accidentally adjusted backwards the impeller “pump out” vanes could cause a stuffing box vacuum if they are too close to the back plate.
- If the pump is lifting liquid, the stuffing box is running in a vacuum..
- Condenser hot wells run under a vacuum.
- Evaporators often run under vacuum.
High soak temperatures can cause a seal failure
- This becomes important after the cooling or flushing is shut off when the pump is stopped. Heat transfer oil experiences this problem,
The shaft material is a variable.
- Some metals are better heat conductors than others. The conductivity of stainless steel is poor compared to carbon steel as an example.
The stuffing box design is important.
- The inside diameter should be as large as possible.
- A steady flow through the stuffing box is very important.
- Suction or discharge recirculation is necessary if you do not want to flush the product with a clean lubricant.
- The shape of the stuffing box can determine the amount of heat being generated. Tapered boxes can cause a rapid rotation of the fluid at the seal faces increasing the amount of heat being generated.
- The material of construction is another variable.
- Is there a gasket between the stuffing box and the back plate? Gaskets do not conduct heat very well.
- Product build up on the inside diameter of the stuffing box acts as an insulator.
- Is there a heating/cooling jacket installed on the stuffing box? Is it clean?
- You need a thermal bushing installed in the end of the stuffing box to isolate the product temperature when you are using a heating or cooling jacket.
- Can the stuffing box be vented in a vertical application? If not you will trap air, and the faces will run dry.
- Has the outside diameter of the stuffing box been insulated? Decide if this is a good idea for your application.
The loss of an environmental control can increase the stuffing box temperature. There are several types of environmental controls in common use:
- Flush. Clean liquid is pumped into the product, diluting it by the amount pumped in.
- Quench. Steam or water is injected behind the seal to wash away any thing that came across the faces and to keep the seal faces at some pre-determined temperature.
- Barrier fluid. High pressure fluid circulated between dual seals.
- Buffer fluid. Low pressure fluid circulated between dual seal faces.
- Jacketing fluid. Is circulated through the stuffing box jacket. Steam and condensate are the two most popular choices. Calcium in the jacket can restrict the heat transfer. Make sure the jacket is clean and the inside of the stuffing box is insulated from the pumpage by a thermal bushing.
- Suction recirculation. installed between the bottom of the stuffing box and the suction piping. This should be the normal stuffing box circulation method.
- Discharge recirculation. Connected between the discharge side of the pump and the stuffing box. It is used to pressurize the stuffing box to prevent a fluid from vaporizing.
The pump discharge is being throttled for some reason causing the heating problem. Here are a couple of reasons why people throttle the pump discharge. :
- To control flow. The pump is too big for the application.
- To stop cavitation. The higher the capacity, the more NPSH needed.
- To create a false head and move the pumping point closer to the best efficiency point.
- If two pumps are connected in series and the first pump has a higher capacity, it will be throttled by the second pump.
- Throttling can occur if the pumps are installed in parallel and one of them is putting a back pressure on the other one.
A change in the process can cause the additional heat.
- A higher process temperature.
- The pump is running at a higher speed.
- A high temperature cleaner or solvent is being used to clean the lines.
- A different fluid is being pumped through the lines.
Here are a couple more reasons the temperature can change in the stuffing box:
- Rubbing of a component because of installation error, shaft deflection or thermal growth.
- The wear rings are contacting because of shaft deflection.
- A protruding gasket or fitting is rubbing a seal part.
- The impeller clearance too small.
- The impeller out side diameter is too close to the cutwater.
- Too much impeller to volute clearance can cause heat to be generated as the fluid is recirculated in the volute.
- Too high an impeller suction specific speed number can cause internal recirculation problems.
Elastomers. Especially those located in or close to the seal faces.
- They will take a compression set at high temperature.
- At higher heat they will shrink, harden and then crack.
- A rubber bellows can rupture.
- At cryogenic temperatures elastomers harden and stop sealing.
- A filler or binder can melt.
- Blisters can form on the seal face as air trapped in the carbon expands because of the face heat. Face pitting occurs when the blisters explode.
- Slip stick can cause chipping of the carbon outside diameter.
- The part can go out of flat due to differential expansion of the various cross sections.
- If the carbon was lapped at room temperature it can go out of flat at temperature extremes.
The hard face
- Thermal cracking is common in some ceramics.
- Heat check is a problem with plated or coated hard faces. It is caused by dissimilar materials expanding at different rates .
- The hard coating or plating lifts off if the sealing fluid attacks the base material. Coating are porous. High face heat will accelerate the process.
- The hard face can go out of flat due to differential expansion of the various cross sections.
The metal parts
- Critical dimensions and surface finishes can change.
- A metal bellows can lose strength as the metal approaches its annealing temperature.
- Heat accelerates any corrosion problems caused by the product or any chemical used to clean the system.
The product can change from a liquid to a solid or gas with higher temperatures.
- It can vaporize and open the lapped faces allowing solids to penetrate..
- It can become more viscous. Some products increase in viscosity as the become cold, others when they get hotter.
- It can solidify. A change in temperature is enough. Some products solidify with heat, others with cold.
- A film can build on the hot seal surface restricting movement. This can occur at the lapped faces or in-between sliding components that must be free to flex and move. This is a problem with hot oil applications
- The product can crystallize with a change in temperature.
- Vaporization at the seal faces can freeze the moisture outboard of the seal or any oil or lubricant that might have been put on the lapped faces.
- The product can lose its lubricating qualities and cause “slip stick” problems. Hot water experiences this failure.
Critical dimensions change that can affect seal performance.
- Alignment between the pump/driver
- Stuffing box squareness to the shaft.
- Pipe strain will increase with temperature changes.
- The wear ring clearance will change.
- The O-ring squeeze will change.
- The interference fit between the face outside diameter and its metal holder can loosen.
Corrosion always increases with an increase in product or face temperature. There are many kinds of corrosion affecting the metal parts that include:
- Overall or general
- Concentrated cell
- Stress. Chloride stress corrosion is a big problem with stainless steel springs and bellows.
- Selective leaching