Packing For Pumps
When selecting packing for an application, we start with the acronym S.T.A.M.P.S. This stands for size, temperature, application, media, pressure and surface speed. Let’s take a closer look at these criteria.
Size is the size of the packing. Packing comes in both English and metric sizes. English sizes start at 1/8” by 1/8” and go up to 1 ½” by 1 ½” or more. Smaller sizes come in 1/16” increments, larger sizes come in 1/8” increments. The size of the packing can be calculated by measuring the stuffing box inside diameter, subtracting the shaft (or sleeve) diameter and dividing by two. The actual size of the packing is a bit larger than the nominal size in order to provide some squeeze for sealing purposes.
Temperature is the maximum temperature the packing will see. This is not the average temperature in the stuffing box or the discharge temperature of the pump but the temperature the packing will experience doing its work in the stuffing box. Friction can add 100 degrees F or more. You can measure the maximum temperature with a temperature gauge with a tell-tale or checking a temperature gauge periodically or measuring with an infrared device.
This application data point refers to whether you want to pack a centrifugal or reciprocating pump, pack a valve, use it as a door seal, for a slide joint or other uses. Will this be a static application or a dynamic application?
What is the process fluid or gas? What other fluids or gases are present? What traces are there going to be? What is the unit cleaned with? In other words, what is every liquid or gas the packing will see. It has to stand up to everything it will see and even traces of a product can destroy packing. You want to know if a liquid has solids in it. What percent of what sizes will be present. This is called the “distribution” of solids. Abrasive liquids are common. Even drinking water has some solids in it. You want to know the maximum range of pH to select the correct packing, too.
Like temperature, you want to know the maximum pressure the packing can see. Do blockages occur where the pressure can spike? The packing has to stand up to this pressure.
This is the circumference of the shaft or sleeve in feet times the RPM. This will give you a number in feet per minute.
In addition to S.T.A.M.P.S., it is important to know the condition of the equipment. Is the stuffing box, shaft or sleeve worn? Does it need to be replaced or reconditioned? If it does you can get by sometimes by using packing one size larger. This is not ideal but can buy you time.
Packing comes in square-braid, interlocked-braid, braid-over-braid, braid-over-core and others.
Square braid is softer and can conform to worn equipment a little better. It holds more initial start-up lubrication, too. However, if one strand gets cut, square-braid packing can unravel. Square-braid packing actually comes out of the braiding machines almost round. It is then rolled, called calendared, to be square.
Interlocked-braid packing is made on newer machines that can weave more stands at one time. Each strand actually passes through the middle of the packing. It is more firms so it supports, or bears, better than square-braid. It is woven square to begin with. It holds less break-in lubrication than square-braid but has more packing material in it. If one strand is cut, the packing does not tend to unravel.
Braid-over-braid and some braid-over-core packings are used for steam valve applications. The outer jacket seals and the inner section blocks the flow of product. Some jackets have metal wires woven in it for strength against the pressure.
In recent years, some packings have been developed specifically for mixers, agitators and pumps with more shaft movement than desired. These packings are woven over various rubber materials with solid or hollow cores to get some recovery or resiliency to prevent a moving shaft from beating the packing out of shape and creating a path of leakage.
Packing materials can be vegetable, acrylic, aramid, PTFE, carbon, graphite, carbon-filled-PTFE or metal.
- Vegetable fibers include ramie, a vegetable fibers that holds up rather well in water. Vegetable fibers are often grease lubricated.
- Acrylic fibers are also called polyacrilonitrile or PAN. Acrylic fibers are usually lubricated with graphite or PTFE.
- Aramid fibers are very strong. Kevlar ™ is an aramid fiber. It is used in applications where there is abrasive solids are present.
- PTFE is used in applications where there are very high or low pH’s present and for food, beverage and dairy applications.
- Carbon and graphite packings include graphite filament and flexible graphite and are used where the process temperature exceeds 400 degrees F.
- Carbon-filled PTFE is black in color and is a popular general purpose packing as its pH and temperature range is broad. It is also less harsh on shafts and can seal on a less-than-idea surface.
- Metal packings such as copper are often used as a bushing to support or bear the weight of the shaft. Hardened shafts are required when using metallic packing.
- Some packings are a combination of fibers. Some have Kevlar on the corners to prevent tearing with a less harsh material in the body of the packing.
The information below is a guide only. Please consult your fluid sealing specialist for specific recommendations. OnHandSupply.com does not make packing recommendations.
- BlackPak. BlackPak is a flake-graphite lubricated acrylic packing. It is the least expensive packing we offer and is suitable for general service on rotary and reciprocating equipment. Its service limits are shaft speeds to 1885 FPM, temperatures to 450 degrees F or 232 degrees C and pH ranges between 4-10. It is black in color.
- WhitePak. WhitePak is coated with a PTFE suspensoid. The fibers are acrylic. Like WhitePak, BlackPak is good for general service on rotary and reciprocating equipment. Applications should fall within shaft speeds up to 1885 FPM, temperatures under 500 degrees F or 260 degrees C and within the pH range of 3-11. It is white in color.
- GeneralPak. GeneralPak is a carbon-filled PTFE fiber that is good for almost all rotating and reciprocating shafts. It can handle any application where carbon and PTFE are suitable. It works with shaft speeds up to 4400 FPM, temperatures to 550 degrees F or 287 degrees C and the entire 0-14 pH range. It is black in color.
- SlurryPak. SlurryPak has Kevlar corners and a carbon-filled PTFE body. It is less harsh on shafts and sleeves than pure Kevlar and dissipates heat better. It is used in slurry or abrasive applications, especially in paper mills where graphite is suitable. It handles shaft speeds to 2500 FPM, temperatures to 500 degrees F or 260 degrees C and 3-11 pH. It is black in color with gold corners.
- GraphPak. GraphPak is for high temperatures. This graphite filament packing can handle temperatures up to 1200 degrees F or 649 degrees C in steam, 800 degrees F or 427 degrees C in oxidizing atmospheres and 0-14 pH. It is not recommended for fuming nitric acid, oleum or fluorine. It is black in color.
- ChemPak. ChemPak is woven from PTFE fibers and is suitable for all oxidizers and alkalis but not for molten alkali metals. It can handle surface speeds to 1202 FPM and 0-14 pH. It is white in color.
- FoodPak. FoodPak is also PTFE fibers but woven on a machine dedicated to producing packing for food serve and cleaned in accordance with FDA proscribed methods. It has the same capabilities of ChemPak and is also white in color.