Q. What is a steam pump? How does it work and where is it used?A. Properly called a "Direct Acting Steam Pump," it is a reciprocating (steam) engine and a pump liquid end built together as a unit. Although steam is implied as the driving medium, compressed gases such as air or natural gas can be used.
Figure 8.2 shows the cross section of such a pump. The steam or driving end is on the left, and the liquid or pump end is on the right. Steam is admitted to the driving end through a sliding valve on its top, which moves back and forth in response to the piston drive rod through the rocker arm, thereby causing reciprocating action of the pump rod. The liquid end acts the same as any reciprocating pump.
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The pump shown is a double acting pump. That is, it pumps water in both directions of the stroke. It can also be built as a duplex unit, which has two pistons driven by two steam cylinders. Duplex units have a more steady pressure and rate of flow and are often preferred. More detail on steam pump construction can be found in ANSI/HI 8.1-8.5 Direct Acting Steam Pumps, available at www.pumps.org.
Direct acting steam pumps were first designed in 1840 by Henry R. Worthington to act as boiler feed pumps aboard early steamboats. Today, they are still used in hazardous applications where electrical power might ignite explosive gases. In addition, they should also be considered where pressurized steam or gas is readily available and might otherwise be wasted. Some manufacturers still produce steam pumps, but they have mostly been replaced by air operated pumps.
Q. Is there a guideline for decontamination of pump parts when it is necessary to send them out for repair? We operate a chemical plant with over 1000 pumps and often find this necessary.
A. Yes. The Hydraulic Institute standard ANSI/HI 9.1-9.5 Pumps - General Guidelines includes section 9.5 that covers that. Key parts of the procedure are as follows:
The complete disassembly, cleaning and decontamination of the pump parts must take place prior to shipping. An itemized list of all of the parts shall accompany the shipment. Cleaning steps to include:
- The product shall be drained and decontamination fluids flushed from the pump/parts.
- The pump/parts shall be dried and free of contaminates, liquid or particulates.
- Radioactive material will not be accepted even if decontaminated or cleaned.
Prior to shipping, prepare two copies of a data pack that includes:
- A statement that decontamination took place.
- A description of the decontamination procedure.
- A signature, with date, of the person certifying that the decontamination was completed.
- A description of the installation and the operation of the pump.
- Description of the liquid characteristics (name, concentration, temperature, pressure, viscosity, particulate, etc.).
- The material safety data sheet (MSDS) for the liquid.
Attach a copy of the data pack pump/parts being shipped. Send a copy to the recipient as backup.
For more detail on this procedure, see ANSI/HI 9.1-9.5.
Figure 12.24
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Q. What should we look for when buying pumps for slurry applications? The literature is quite confusing with alternatives including hard metals of different types as well as soft elastomers.
A. Selection of the best design and materials of construction for centrifugal pumps for slurry service is indeed a complex problem and defies any simple suggestion that can be made here. However, for the first step in the process, you may consider the following two figures.
Figure 12.24 classifies the slurry based on its particle size, concentration and specific gravity. Slurries in Class 4 are the most difficult to pump considering erosive wear.
Table 12.8 shows the suitability of different wetted materials for resisting wear.
Mechanical design is dictated by the material of the wetted parts. Cast iron and stainless steel usually do not require any special design. White irons and similar very hard materials are difficult to machine and are also brittle. For this reason, they are clamped together instead of being bolted. Elastomers are usually made as replaceable liners that are clamped inside the casing and cover. Impellers are bonded with the elastomeric materials.
Table 12.8. General suitability of wetted materials.
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Pumps & Systems, May 2007