Pump Handbook
Solids and Slurries - Definitions & Applications (Cont.)
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Solids and Slurries - Slurry Pump ApplicationsDetermining when to use a slurry style centrifugal pump can be a challenging decision. Often the cost of a slurry pump is many times that of a standard water pump and this can make the decision to use a slurry pump very difficult. One problem in selecting a pump type is determining whether or not the fluid to be pumped is actually a slurry. We can define a slurry as any fluid which contains more solids than that of potable water. Now, this does not mean that a slurry pump must be used for every application with a trace amount of solids, but at least a slurry pump should be considered. Slurry pumping in its simplest form can be divided into three categories: the light, medium and heavy slurry. In general, light slurries are slurries that are not intended to carry solids. The presence of the solids occurs more by accident than design. On the other hand, heavy slurries are slurries that are designed to transport material from one location to another. Very often the carrying fluid in a heavy slurry is just a necessary in helping to transport the desired material. The medium slurry is one that falls somewhere in between. Generally, the percent solids in a medium slurry will range from 5% to 20% by weight. After a determination has been made as to whether or not you are dealing with a heavy, medium, or light slurry, it is then time to match a pump to the application. Below is a general listing of the different characteristics of a light, medium, and heavy slurry. Light Slurry Characteristics:
Medium Slurry Characteristics:
Heavy Slurry Characteristics:
The previous listing is just a quick guideline to help classify various pump applications. Other considerations that need to be addressed when selecting a pump model are:
The designers of slurry pumps have taken all the above factors into consideration and have designed pumps to give the end user maximum expected life. Unfortunately, there are some compromises that are made in order to provide an acceptable pump life. The following short table shows the design feature, benefit, and compromise of the slurry pump.
Although selecting the proper slurry pump for a particular application can be quite complex, the selection task can be broken down into a simplified three-step process:
Slurry pumps can be broken down into two main categories. The rubber-lined pumps and the hard metal pump. However, because of the elastomer lining, the rubber-lined pumps have a somewhat limited application range. Below is a general guideline which helps distinguish when to apply the rubber-lined pumps.
It should be noted, however, that a hard metal pump can also be used for services that are outlined for the rubber-lined pump. After a decision has been made whether to use a hard metal or a rubber lined pump, it is then time to select a particular pump model. A pump model should be selected by reviewing the application and determining which model pump will work best in the service.
NOTES: The model HS pump is unique pump in that it is a recessed impeller or "vortex" pump. This style pump is well suited to handle light pulpy or fibrous slurries. The recessed impeller used in the HS family of pumps will pass large stringly fibers and should be considered when pump plugging is a concern. The Model AF is a specialized pump with an axial flow design. This design of pump is built specifically for high flow, low head applications. In general, slurry pumps have been designed to handle fluids with abrasive solids and will give extended lives over standard water or process pumps. Although many features have been designed into the slurry pump, there are still two factors which directly relate to the pump's life that can be determined. The first choice to make is determining the metallurgy of the pump. In most cases, a hard metal slurry pump will be constructed of some hardened metal with a Brinell hardness of at least 500. Goulds standard slurry pump material is a 28% chrome iron with a minimum hardness of 600 Brinell. This material is used for most abrasive services and can also be used in some corrosive fluids as well. If a more corrosion-resistant material is required, then the pump may be constructed out of a duplex stainless steel, such as CD4MCu. Please check with your nearest Goulds sales office if you are unsure what material will be best suited for a particular application. PUMP RUNNING SPEEDThe other factor that can be controlled by the sales or end user engineer is the pump running speed. The running speed of a slurry pump is one of the most important factors which determines the life of the pump. Through testing, it has been proven that a slurry pump's wear rate is proportional to the speed of the pump raised to the 2 1/2 power. 
With the above ratio in mind, it can be shown that by cutting a slurry pump speed if half, you get approximately 6 times the wear life. For this reason, most slurry pumps are V-belt driven with a full diameter impeller. This allows the pump to run at the slowest possible running speed and, therefore, providing the maximum pump life. WHY USE A V-BELT DRIVEIn most ANSI pump applications it is a reasonable practice to control condition point by trimming the impeller and direct connecting the motor. However, this is not always sound practice in slurry applications. With abrasive solids present, wear life is enhanced by applying the pump at the slowest speed possible. Another situation where V-belts are beneficial is in the application of axial flow pumps. Axial flow pumps cannot be trimmed to reduce the condition point because they depend on close clearances between the vane tips and the casing for their function. The generally low RPM range for axial flow application also makes it beneficial to use speed reduction from the point of view of motor cost. The types of V-belt drives available for use in pump applications are termed fixed speed, or fixed pitch, and variable speed. The fixed pitch drive consists of two sheaves, each machined to a specific diameter, and a number of belts between them to transmit the torque. The speed ratio is roughly equal to the diameter ratio of the sheaves. The variable speed drive is similar to the fixed speed except that the motor sheave can be adjusted to a range of effective or pitch diameters to achieve a band of speed ratios. This pitch adjustment is made by changing the width of the V grooves on the sheave. Variable speed drives are useful in applications where an exact flow rate is required or when the true condition point is not well defined at the time that the pump is picked. V-belt drives can be applied up to about 2000 horsepower, but pump applications are usually at or below 350 HP. |
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