Part 6: Between Bearings, Axially Split Pumps
DuPont™ Vespel® CR-6100 is a composite material which is used for the stationary wear parts of your pump and can be used in nearly all process chemicals from cryogenic temperatures to 500 F (260 C).
Upgrading your pumps with Vespel® CR-6100 wear parts allows you to eliminate the metal-to-metal contact points in the pump and reduce the clearance at the wear parts. This simple change allows you to minimize the risk of pump seizure and helps to improve pump reliability and efficiency.
Axially Split Pumps
Today, we cover the details on using Vespel® CR-6100 to upgrade your between-bearings, axially-split pumps. The components we want to upgrade in these pumps are the case rings, center bushing, throttle bushing, and throat bushings.
Single and two-stage BB1 style pumps are often used in product transfer, water, or low NPSH applications. Upgrading the wear rings (and inter-stage bushing in two-stage designs) with Vespel® CR-6100 will help maintain rotor stability and improve efficiency. Reducing wear ring clearance can also help reduce the pump NPSHR
Multi-stage BB3 designs are used in several ideal applications for Vespel® CR-6100: boiler feed water, naphtha charge, amine stripping, and product pipeline and shipping applications. These pumps are designed with inherently flexible rotors which rely upon the Lomakin Effect for rotor stability. Upgrading these pumps with Vespel® CR-6100 and reducing the clearance can reduce vibration levels and make your pump more reliable.
These designs also experience significant efficiency gains when you reduce the clearance at the wear rings, center bushing, and throttle bushing. Most of the multi-stage designs will show at least 3-4% efficiency gain and several customers have reported efficiency gains in excess of 5%.
Solid or Insert?
The first question to address with an axially split assembly is whether or not to make the components out of solid Vespel® CR-6100 or to use Vespel® CR-6100 as an insert into a metal holder (figure 1).
Consider that Vespel® CR-6100 has a coefficient of thermal expansion which is about 60% less than carbon steel in the radial plane. In a hot pump, the metal parts are going to thermally expand more than the Vespel® CR-6100 parts. We compensate for the thermal expansion difference by installing the Vespel® CR-6100 with a press fit into a metal holder. As the metal holder expands, part of the press fit is relieved, and the Vespel® CR-6100 “follows” the growth of the metal parts.
Therefore, if your pump operating temperature is > 140 F (60 C), the Vespel® CR-6100 needs to be installed as an insert into a metal holder to maintain the clearance up to pump operating temperature. If it is an ambient temperature application, you can install Vespel® CR-6100 as a solid component or as an insert, whichever is easier for you.
One advantage of using inserts in these pumps is that during a repair, you can often salvage the used metal parts, machine the bores and reuse the old parts as holders for the Vespel® CR-6100 inserts. This can save time and reduce the cost of upgrading the pump with Vespel® CR-6100.
Another feature unique to axially split pumps is that the center bushings and inter-stage rings might also be axially split (Figure 2).
When the parts are split, we apply the same rule concerning solid or insert parts based on operating temperature. If the pump is operating at ambient temperature, the parts can be manufactured from solid Vespel® CR-6100; if the pump is operating at elevated temperature, we need to install the Vespel® CR-6100 as an insert with a press fit.
The press fit for split parts is achieved by modifying the metal components such that they bolt together. The bolting force is sufficient to create the press fit of the Vespel® CR-6100 component.
If you need to fabricate split parts, please contact Boulden and we will send you specific instructions based on your application details.
Center and Throttle Bushings
Another unique characteristic of multi-stage axially split pumps is that the center and throttle bushings can be exposed to high differential pressures. For these components, Boulden has developed a patent-pending design called the PERF-Seal™ (also shown in Figure 2) which improves the performance and dramatically increases the differential pressure capability of composite materials used in these positions.
Boulden recommends the PERF-Seal™ design for all center and throttle bushings in multi-stage pumps. Contact us and we can either supply machined parts or provide drawings for you to manufacture the components in your shop for the upgrade.
Until next time…
In the next months, we will discuss radially-split between bearings pumps and vertically suspended pumps.
Until then, if you have an axially split pump where you would like to improve the reliability or efficiency, contact Boulden today. We can provide you all of the details required for your upgrade and have the Vespel® CR-6100 in stock in a wide range of sizes available for immediate delivery.
For details on installing Vespel® CR-6100 into nearly any centrifugal pump type, download the Boulden Installation Guide.
Chris Froome in Mondorf Les Bains, Luxembourg, ville-départ, Tour de France, Stage 4 on July 4th, 2017. Walking distance from Boulden’s European Office