Modern, Large Centrifugal Pumps Provide Reliability

Considering the pump and complete system can decrease unplanned downtime.

Written by:
Amin Almasi
Published:
September 27, 2013

Centrifugal pumps are a popular choice in many plants because they are simple and reliable, and have a light-weight and compact design. Increased use of centrifugal pumps in many applications, such as process applications, in recent decades has occurred for four reasons:

  • Advances in the centrifugal pump seal technology
  • Modern hydrodynamic and rotordynamic knowledge and modeling
  • Advanced manufacturing methods to produce accurate rotating parts and complex components with reasonable costs
  • The ability to simplify the control through the use of modern control technology, particularly modern variable speed drives (VSDs)

Centrifugal pumps do not experience the internal shaking and complex pulsation problems of reciprocating pumps. Therefore, they do not need the same large foundation or have the everyday pulsation problems and component repairs. As plant sizes increase, the pressure to improve reliability is high because of the large economic impact of unplanned downtime. In many large process plants, the impact of nonscheduled shutdowns is much greater than the long-term impact of a small decrease in efficiency (this refers to the lower efficiency of centrifugal pumps compared to positive displacement pumps).

Pump Configuration

Horizontal split casing is commonly used for low- and medium-pressure applications. Large numbers of horizontal split-case centrifugal pumps are installed in petrochemical plants, refineries, water treatment plants and other process plants.

Horizontal split-case pump maintenance is simple and straightforward. To maintain a proper joint seal when the pressure is high, a vertically split (barrel type) pump is used. This article focuses on horizontal split-case centrifugal pumps. Cast iron could be used for low-pressure pumps. For flammable or toxic process liquid services, a suitable steel grade is the minimum requirement. For pump casings, cast steel or fabricated casing should be used. Casings may also be heat treated depending on the thickness, fabrication details, applicable codes and the pump service.

When applications are complex and cannot be accommodated by a single-case pump, multiple cases can be used. A popular configuration is the tandem-driven series arrangement using a common driver. Gear units may be included in a pump train, either between the casings or between the driver and the pump casings. The maximum number of pump casings is usually three. Longer, tandem-driven connected pump trains tend to encounter specific speed problems.

A double-flow pump arrangement could be used for some applications. At the inlet, the liquid stream is divided into parallel steams, and the volume is reduced to a value within the specific capability of a single-flow pump. The pump casing bolts require attention. Proper pre-loading of the casing bolts is necessary to prevent unloading because of the cyclic operation.

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Component Design

Pump shafts should be made from one piece of heat-treated, forged low-alloy steel, suitably ground (forged as close as possible to the final dimensions). Forged low-alloy steel shafts are standard shafts for large centrifugal pumps in process industries. Only pumps that handle highly aggressive liquids require corrosion-resistant shafts. Shaft sleeves are frequently fitted so that the sealing elements do not operate directly against the shaft.

Two types of impellers—a closed impeller (consisting of a hub, blades and a cover) and a semi-open impeller (consisting of a hub and blades) are commonly used. Open impellers are available but rarely used.

The smallest possible pump that meets the application’s specifications is usually preferred. This leads to impellers with high flow coefficients. For a multistage pump, the first stage impeller should have the maximum flow coefficient. In multi-casing, high-pressure pump trains, the first pump casing—because it has the highest suction volume—will dictate the train speed. This will lead to sub-optimum designs as the suction volume of the later stages decreases (particularly in high-pressure services in which the liquid behaves like a compressible fluid). The solution could be a gear unit between the casings for large pressure ratios. This is an advantage of using a higher shaft speed at higher pressures (where the volumetric flow is smaller).

The head generated is fixed by the pump impeller’s dimensions—particularly by the exit angle, tangential tip speed and the slip. The head at the delivery, because of internal losses, also depends on the flow rate. However, the head required is determined by the process conditions (downstream facilities). Identifying all the process duties to be met by a centrifugal pump before the pump order (the pump selection and the design freeze) is important. Once the pump’s design and dimensions are fixed, duties other than those specified may not fall within the pump operating range or can only be accommodated, if at all, by inefficient operation.

For a minimum capital cost, the maximum permissible tip speed is often selected. However, this may lead to a narrow operation range. A wider range and a higher efficiency can be obtained if the tip speed is slightly reduced (5 to 15 percent). The number of impellers needed to achieve a given head could be increased by the reduction of the tip speed, but this solution is preferred.

Manufacturers generally use standard designs arranged in a series of shapes and sizes. Each family of pump impeller covers a range of flow coefficients. The number of impellers that can be accommodated in one pump casing is dictated by rotordynamic considerations.

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