Interstage Construction for Axially Split Casing Pumps

By 14 January 2017

A multistage pump inherently has adjoining chambers subjected to different pressures, and means must be made available to isolate these chambers from one another so that the leakage from high to low pressure will take place only at the clearance joints formed between the stationary and rotating elements of the pump. Thus, the leakage will be kept to a minimum. The isolating wall used to separate two adjacent chambers of a multistage pump is called a stage piece, diaphragm, or inter-stage diaphragm. The stage piece may be formed of a single piece, or it may be fitted with a renewable stage piece bushing at the clearance joint between the stationary stage piece and the part of the rotor immediately inside the former. The stage pieces, which are usually solid, are assembled on the rotor along with impellers, sleeves, bearings, and similar components. To prevent the stage pieces from rotating, a locked tongue-and groove
joint is provided in the lower half of the casing. Clamping the upper casing half to the lower half securely holds the stage piece and prevents rotation.

The problem of seating a solid stage piece against an axially split casing is one that has given designers much trouble. First, there is a three-way joint and, second, this seating must make the joint tight and leakproof under a pressure differential without resorting to bolting the stage piece directly to the casing.

To overcome this problem, it is wise to make a pump that has a small-diameter casing so that when the casing bolting is pulled tight, there is a seal fitting of the two casing halves adjacent to the stage piece. The small diameter likewise helps to eliminate the possibility of a stage piece cocking and thereby leaving a clearance on the upper-half casing when it is pulled down. No matter how rigidly the stage piece is located in the lower-half casing, there must be a sliding fit between the seat face of the stage piece and that in the upper-half casing so that the upper-half casing can be pulled down. Each stage piece, furthermore, must be arranged so that the pressure differential developed by the pump will tend to seat the piece tightly against the casing rather than open up the joint.

We have said that axially split casing pumps are typically used for working pressures of up to 2,000 lb/in2 (138 bar). High-pressure piping systems, of which these pumps form a part, are inevitably made of steel because this material has the valuable property of yielding without breaking. Considerable piping strains are unavoidable, and these strains, or at least a part thereof, are transmitted to the pump casing. The latter consists essentially of a barrel that is split axially, flanged at the split, and fitted with two necks that serve as inlet and discharge openings. When piping stresses exist, these necks, being the weakest part of the casing, are in danger of breaking off if they cannot yield. Steel is therefore the safest material for pump casings whenever the working pressures in the pump are in excess of 1,000 lb/in2 (70 bar).

This brings up an important feature in the design of the suction and discharge flanges. Although raised-face flanges are perfectly satisfactory for steel-casing pumps, their use is extremely dangerous with cast-iron pumps. This danger arises from the lack of elasticity in cast iron, which leads to flange breakage when the bolts are being tightened, the fulcrum of the bending moment being located inward from the bolt circle. As a result, it is essential to avoid raised-face flanges with cast-iron casings as well as the use of a raised face flange pipe directly against a flat-face cast-iron pump flange. Suction flanges should obviously be suitable for whatever hydrostatic test pressure is applied to the pump casing.

The location of the pump casing feet is not critical in smaller pumps operating at discharge pressures below 275 lb/in2 (19 bar) and at moderate temperatures of up to 300°F (150°C). Since the unit is relatively small, very little distortion is likely to occur. However, for larger units operating at higher pressures and temperatures, it is important that the casing be supported at the horizontal centerline or immediately below the bearings (refer to Figures 19 and 20).