Issue No. 251〔Special edition〕
Under the Scenes of our Lives High-pressure Pump
- Technologies and Structure -
Author
Tadashi KONDO
Fluid Machinery & Systems Company
In order for a high-pressure pump to operate stably in the place where it should play its role, it must provide an excellent balance and endure high pressures. These requirements are specific to high-pressure pumps. High-pressure pumps are based on technologies for achieving these requirements. This article outlines these technologies and how they are related to the structure of high-pressure pumps.
Basic structure of high-pressure pumps
The structure of high-pressure pumps is based on two technologies: one for achieving an axial thrust balance, a requirement specific to multi-stage pumps, and the other–double casings–for enduring higher pressure. Here, let me explain these technologies in some detail.
While rotating in liquid, an impeller produces centrifugal force, which increases the impeller outlet pressure (head). The increased pressure produces axial thrust (axial load) according to the size of the impeller.

Fig. 1 Structure of a boiler feed pump (Model HDB)

Fig. 2 Water flow inside a boiler feed pump (Model HDB)

Table Basic types of Ebara's high-pressure pumps
Structure of high-pressure pumps
(1)Axial thrust acts on impellers
An impeller in operation has a section on which suction pressure acts, and a section on which discharge pressure acts; these sections are separated by a wearing part having a narrow gap. For a doublesuction impeller (Fig. 3), thrust that presses the entire impeller leftward is produced by configuring the right wearing diameter to be smaller than the left one so that discharge pressure will act on a larger area. A typical double-section single-stage pump is designed to have the same right and left wearing diameters so that theoretically no thrust will be produced; in some cases, however, thrust is intentionally applied to make the shaft stable. On the other hand, any multi-stage, high-pressure pump has right and left wearing diameter different from each other to allow thrust to be applied to the shaft always in the same direction.
For a single-suction impeller (Fig. 4), discharge pressure (which will act as suction pressure for the next stage) acts on the side opposite to the suction side of the impeller, causing large thrust to be applied toward the left (suction) side.

Fig. 3 Double-suction impeller

Fig. 4 Single-suction impeller
(2)Straight-through arrangement (Model SS and DC)

Fig. 5 Balance of the axial thrust of impellers arranged
in the same orientation

Fig. 6 Operating principles of the balance disc
(3)Back-to-back arrangement (Model SP and HDB)
Since a volute-type, axially-split-casing, multi-stage pump has impellers arranged back to back as shown in Figure 7, the thrust is offset between the right and left groups of single-suction impellers. For the doublesuction first stage, the thrust is calculated based on the concept described in (1) above. For the first-stage sleeve, balance sleeve, center stage piece (all these shown in Fig. 1), single-suction, single-stage impellers in the odd-numbered stages, and the other sections where the thrust is not offset, the thrust is calculated according to the pressure receiving area and differential pressure. The wearing diameters are thus designed so that the thrust is balanced throughout the rotor. Compared with the radially split type mentioned before, the shaft of the axially-split-casing type is relatively easy to design because the thrust that acts on the shaft is decentralized and no balance disc is required.
Why double casings are required
In a pump that uses double casings, the inner casing is confined inside the outer casing so that the outside of the inner casing is filled with the discharge pressure of the last pump stage to allow the external pressure to act on the inner casing. This enhances the strength of the split surface to ensure the seal. This structure is adopted in model HDB (Figure 9). After the internal element (an assembly of the inner casing and the rotor) is inserted into the outer casing, a thick cover is installed and the gaskets are fastened with a dozen or more bolts (Figure 10). The outer casing is a simple cylinder that withstands the pressure that a single casing cannot withstand in terms of design. A variation of model SS, single-case, radially split pump, that uses double casings instead of a single casing, is model DC.

Fig. 7 Axial-thrust balance of the impellers arranged back to back

Fig. 8 Example of arrangement of casing bolts for Model SPD
(for withstanding pressures up to 15 MPa)

Fig. 9 Longitudinal section of Model HDB

Fig. 10 Model HDB (with an outer casing that withstands
pressures up to 55 MPa)
Conclusion
In this short article we introduced the high-pressure pumps associated with our everyday life in the first half and the basic structures of high-pressure pumps in the second half. I hope that you now understand highpressure pumps to some extent.
I am confident that Ebara’s high-pressure pumps are on the world’s top level in any area. This status was achieved, I believe, through the basic technologies described in this article as well as our detailed consideration and expertise about the design and manufacturing of pumps and our customers who highly rate the performance of our pumps. It is also the result of our long experience as well as occasional collaboration with our customers.
As an engineer, I hope that Ebara, not content with the status-quo, will support the next generation society by continuing to manufacture high-pressure pumps toward further development of the product.
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