Solid Shaft Designs & Cartridge Seals

This month's column focuses on whether to use shaft sleeves in overhung centrifugal pumps (Type OH-1 per American Petroleum Institute [API] 610 designation). The most common OH-1 pump type is the American National Standards Institute (ANSI) B73.1M. The practice of using cartridge-type mechanical seals on solid pump shafts (in lieu of sleeved shafts) is not new or radical. While the benefits of building a pump in this manner are real and measurable in most instances, a large percentage of pump owners will not change.

Background

I would estimate that shaft sleeves have been used on pump shafts for at least 100 years. My 1919 edition of Pumping Machinery by Arthur M. Greene mentions "sacrificial shaft liners." I do not know exactly when the first pump shaft sleeve was put into service, but I assume it was not long after someone adjusted the packing incorrectly on an expensive pump. Shaft sleeves serve multiple purposes. The most important is to protect the main pump shaft from wear caused by packing abrasion, followed closely by prevention of erosion and corrosion. In some pump designs, the sleeve serves additional purposes. For example, in some horizontal split-case pumps, the sleeve also serves (in conjunction with a threaded shaft nut) as an adjustable means to axially locate the impeller on its respective mechanical and hydraulic center in the casing. The sleeve is designed to be the inexpensive and replaceable part. It is easier and less expensive to change a sleeve than the whole shaft. Users who have packing in their pump consider the sleeve a must-have design feature. In 1905, the mechanical seal as we know it was invented, but it was not commonly used until after World War II. Fifty years ago, most centrifugal pumps in industrial and commercial services still had packed stuffing boxes. Pumps with mechanical seals were uncommon and expensive. Later in the 1970s, many users began to use simple mechanical seals because of safety concerns, stricter Environmental Protection Agency (EPA) regulations, and the cost of both lost product and flush fluids. These simple seals were eventually displaced by component type mechanical seals. Prudent pump users continued to use the existing shaft sleeve designs because the component seals were held in place by tightening a number of set screws. The torqued set screw points damaged the shaft sleeve surface. These "dog marks" (damaged metal surfaces) from the set screws were an accepted negative side effect because of the shaft sleeve's status as an inexpensive and replaceable part. In recent years, most pump users have switched to cartridge-type mechanical seals. Most present-day designs will not damage the shaft or shaft sleeve during installation, operation and subsequent removal from service. Even O-ring fretting of the shaft or shaft sleeve is eliminated in most new designs. When the American Voluntary Standards (AVS) pumps and the forerunners of the modern ANSI pump (B73.1M) were designed in the late '50s and early '60s, packed stuffing boxes were standard. If these pumps were to be redesigned today, they would use cartridge-type mechanical seals, and the design length of the shaft from the radial bearing to the impeller would be shorter. When a packed pump is operating, the packing acts like an additional line bearing because of the hydrodynamic properties of the close clearances between the shaft sleeve and the packing. This "consequential and beneficial phenomenon" mitigates shaft deflection caused by any unequal radial forces acting on the impeller. Without packing around the shaft (for example, when a seal is used), the shaft will deflect more, especially if the pump is not operating near the best efficiency point/best operating point (BEP/BOP). Pump operation at or near shutoff (far left side of the curve) and at runout conditions (far right side of the curve) away from the preferred operating region results in shaft deflection, which causes premature bearing and mechanical seal face wear, shortening the life of these critical components. The ability of a shaft to resist deflection is a direct function of the overhung length and the shaft diameter. This is commonly referred to as the shaft stiffness ratio, shaft deflection ratio, or the L over D ratio (L³/ D⁴). The lower the ratio number, the better the shaft will resist deflection. The formula for calculating the ratio factor is based on the simple cantilevered beam deflection formula. Many of the factors in the beam deflection formula cancel each other out when applied to an overhung pump shaft. As a result, the revised formula is simply the length (L) of the shaft as measured from the centerline of the radial bearing position to the centerline of the impeller (take L to the third power) divided by the diameter of the shaft in this area (D) to the fourth power, or L³/ D⁴ (see Figure 1). A smaller shaft diameter results in a higher stiffness ratio, which is not a desired attribute. Without delving into a protracted formula derivation, the strength of the shaft material is of little importance, but the modulus of elasticity (Young's modulus) does come into play. Most common shaft materials share similar ranges for the modulus of elasticity. If pump shafts are breaking, the cause is usually cyclic fatigue, not material strength. So a stronger material is not the answer, but preventing or reducing deflection is. When a pump is purposely designed to incorporate a shaft sleeve, the original shaft diameter in the packing and mechanical seal area is typically machined down to a smaller diameter of some incremental distance (D) to accommodate the corresponding sleeve. Some manufacturers' processes design and machine the shaft differently. Either way, the end result is that the shaft has a smaller diameter on the overhung portion. The smaller diameter yields a higher shaft deflection ratio, which means the shaft will deflect more for a given radial force. More deflection will result in deleterious effects on the seal and bearings. Note that the presence of the sleeve does not contribute to the stiffness factor, no matter how tight the fit. The sleeve is not an integral part of the shaft and does not become a factor in the equation. In the past, most pump shafts were generously over-designed/sized to transmit some amount of torque (horsepower) at some speed range, but most shafts were not designed for high side loads (like belt or chain drives) and cyclic fatigue factors. Current designs are taking these side load factors more into consideration.

Current Practices

Many pump owners continue to use old design shaft sleeves when using new design cartridge mechanical seals. There are some good reasons, such as corrosion/erosion mitigation, for continuing this practice. In most cases, however, there is no other reason than "that is the way we have always done it." I would say that, for a given system (curve) and the consequential pump operation on its curve, the pump life would be much longer if the shaft design was solid versus sleeved. The pump would be more reliable, and the mean time between failure and repair (MTBF/R) would be longer. In some modern OH-1 pump models, the incorporation of a shaft sleeve is by design and is an acceptable practice because the shaft deflection ratio is already very low as a result of a generous shaft diameter in the sleeve area. X-17 ANSI pumps (ANSI sizes A105, 110 and 120) are one example (see Figure 2, page 18). Many pump manufacturers also offer solid pump shafts that are made of different materials in the wetted (sacrificial) versus non-wetted areas. Furthermore, most ANSI pump manufacturers offer an optional shaft of larger diameter for given midrange sizes (for example, MT, or medium-sized shaft and bearing systems, versus LT, or large sized, models). Even with sleeve construction, the deflection ratio between the two models of different shaft diameters is significant. The difference between the smaller (MT) sleeved shaft and a larger (LT) solid shaft is dramatic. ANSI B73.1M has set tolerances for allowable shaft deflection at the stuffing box area of the shaft over the allowable operating range. Pumps in compliance with this specification are not allowed more than 0.002 inches of deflection. Simply looking at the L/D ratios is one way to evaluate the pump, but it is equally important to calculate the radial loads for your specific operation and the resultant shaft deflection. Shaft runout, or total indicator reading, with a sleeve design is harder to control because of the added surfaces and associated tolerances. The allowable shaft runout on a solid shaft is 0.001 inches and 0.002 inches for a sleeved shaft. As a final design note, any sleeve design needs to allow for thermal expansion and contraction. Even when we know we should implement change, some habits are hard to break. Are you operating your overhung pumps with cartridge seals and still using shaft sleeves? I would like to hear why.