Choosing Your Upstream Pumping Support System | 2024 | Blog | Resources | John Crane

Choosing Your Upstream Pumping Support System

29 February 2024

8 Minute Read

About Upstream Pumping Technology

Upstream Pumping (USP) is a seal face technology unique to John Crane, which adds the spiral grooves, commonly seen in non-contacting gas seals, to the inboard seal of a dual mechanical seal running on a liquid barrier fluid. The grooves draw in the barrier fluid and pressurize it as it reaches the tips of the grooves. This pressurization has the combined effect of causing the seal faces to ‘lift off’ each other, running with zero contact, and it can cause a low-pressure barrier fluid to leak backward by being pushed into a high-pressure process stream.

More information on this novel technology, including a video showing the operation of the spiral grooves, can be found here.

The non-contacting seal face and unique pumping technology create several benefits for the operator of these seals, including a significantly increased mean time between repair (MTBR) and an ability to survive many service conditions that could cause severe damage to traditional mechanical seals. This blog will explore the sources of damage that are overcome by the unique operation of USP seals.


USP Seal Needs

USP seals have different needs from traditional mechanical seals due to their novel capabilities, meaning that traditional support systems are not optimized to provide for their needs, or monitor their performance.

A USP seal needs a continuous supply of clean fluid at a lower pressure than the process fluid, at a reasonable temperature, and able to respond safely to potential failure modes of USP seals.

John Crane developed the GS USP product to be optimized for the needs of standard USP seals. Due to the variety of applications for USP, two versions were developed, the reservoir version (GS USP-R) and the direct connect version (GS USP-D) While some USP seals can use either version, there will be applications where the correct choice is critical.

Barrier Pressure

While most USP seals can operate with a very low pressure that can be supplied by 1 to 2 meters of fluid height, there are several factors that can lead to higher recommended barrier pressures. These include:

  • Higher temperatures
    • If the pump fluid is at elevated temperature, a water barrier fluid will cavitate or vaporize at lower pressures, so elevated barrier pressures may be required to maintain a liquid state within the seal.
  • Low-speed operation
    • Some pumps operating conditions may include short periods of low-speed operation. For example, in tailings services for the mining industry, some of the pump stages may experience a ‘turbine’ effect which causes inactive pumps to rotate at low speeds. To encourage lift-off at these low speeds, a higher barrier pressure may be used.
  • High seal chamber pressure
    • If the seal chamber pressure in the pump is high while the pump is stationary, a higher barrier pressure can help to prevent seal leakage of process fluid into the support system while the pump is stationary. If the seal chamber pressure is high during running, a higher barrier pressure can reduce the pressure differential across the inboard seal, potentially allowing a lower-rated seal to be selected.

The GS USP-R only provides a 0.1 bar to 0.2 bar (1.5 psi to 3 psi) barrier pressure to the mechanical seal, from the heater pressure of mounting the tank 1m to 2m (3ft to 6ft) above the seal. If a barrier pressure above 0.2 bar (3 psi) is required, it will be extremely impractical to achieve this with header pressure, and a system that can transfer a higher pressure to the barrier fluid is required.

The GS USP-D can supply a barrier pressure up to the pressure of the supply line for the system. The barrier pressures for USP seals are rarely above 6 bar, meaning that the pressure in most water supplies (mains pressure or seal water supplies at mines or paper mills) will normally be able to supply a USP seal without any pressure-boosting equipment required.

The GS USP-D is therefore the correct choice when higher barrier pressures are required. Contact John Crane (link)for more specific advice about the barrier pressure required for your application.

Drinking Water

During normal running, the barrier fluid will be pumped across the inboard seal face into the process stream of the pump, and zero process fluid will contaminate the barrier fluid or the support system.

However, the support system design should account for the possibility of damage to the inboard seal faces or a secondary seal, where the process fluid could enter the seal support system.

If the system is directly connected to a supply, using the correct protection to prevent process fluid from contaminating the supply. Depending on the country/region where the equipment is operating, and the type of supply, different rules/regulations may apply. For example, if the system is supplied by a drinking water system in the EU, standard EN 13077 defines the required backflow protection.

In some areas, or if the barrier fluid is not connected to a drinking water supply, a simple check valve may be sufficient protection.

The best-in-class backflow protection is to have a physical air gap with a minimum height between the supply source and the liquid in the system (usually an AB air gap). This type of backflow protection is usually only practical on systems using atmospheric tanks.

It should be noted that, due to the extreme durability of USP seals, damage to the inboard seals is rare, even after years of service.

The GS USP-R uses an AB air gap to provide a physical gap between the barrier fluid and the supply. Regardless of the potential hazard of the process fluid, the GS USP-R can always be connected to a drinking water supply.

The GS USP-D uses a check valve to provide backflow protection to the supply. In order for a GS USP-D to be connected to a drinking water supply, the exact nature of the process fluid needs to be reviewed and compared against the local regulations for the installation site to protect drinking water from different hazards. The GS USP-D is therefore not normally recommended for drinking water supplies. If the GS USP is required for a drinking water supply due to high barrier pressure requirements, contact John Crane for further advice.

Water Quality

The barrier fluid used by a USP seal must be clean enough to avoid clogging the spiral grooves or springs and avoid abrasive damage to the seal components. If the barrier fluid being supplied to the seal is already free from solids, no filtration will be required.

Both the GS USP-R and the GS USP-D are available with a small filter, at either 10μm or 25μm, which is fine enough to run a USP seal. While the filter is small, the flow through the filter will be equal to the consumption rate of the seal, which is a very small value (up to 500ml/hr) and the filter will be able to run for long periods without cleaning or changing the filter element. Only the dirtiest sources of barrier fluid would require a large filter, and often a small strainer will be sufficient for a long life.

Other Considerations

There are a few other considerations that might create a preference for one type of GS USP over the other.

Mounting location

The GS USP-R is a larger system due to the presence of the tank. In addition, to provide the 0.1 bar to 0.2 bar header pressure (1.5 psi to 3 psi) the tank must be mounted at least 1 meter (3 ft) above the pump shaft centerline, to a max of 2 meters (6 ft).

Some sites may not have the space available above the pump or, if the pump shaft centerline is already high enough above the ground, this could place the GS USP-R at a height that is difficult to reach for maintenance, and to read the instruments to monitor the performance of the system.

The GS USP-D mounting position is not dependent on a minimum height and has a smaller footprint than the GS USP-R. It may therefore be preferable to the GS USP-R at some sites.

Running after loss of supply

The barrier fluid in a GS USP system is supplied from a continuous source which automatically tops itself up. The loss of this supply could cause the system to empty of barrier fluid and the seals to run dry.

To account for this failure mode, a USP system should include either:

  • A large enough quantity of barrier fluid that the system can run for several hours after the barrier fluid supply is lost.
  • A method to detect the loss of the barrier fluid supply and alert the user immediately so that the system can be shut down.

The reservoir of the GS USP-R contains enough fluid for the seal to run for several hours after the supply is lost. The level indicator on the tank would also provide a visual indicator that the system supply had been lost. The GS USP-R can be provided with a low-level switch or transmitter to send an alarm signal to the operator as soon as the level begins to drop.

The GS USP-D does not contain a large reservoir of barrier fluid. If the supply is lost the equipment must be shut down within a few minutes to prevent damage to the mechanical seal. Loss of supply can be visually detected with the pressure gauges fitted to the system. Alternatively, the system can be supplied with low-pressure switches or transmitters to send an alarm signal to the operator as soon as the supply pressure begins to drop.

If the barrier fluid supply is unreliable, and continuing to run for several hours after the supply is lost is a critical feature, the GS USP-R is recommended over the GS USP-D

For further assistance in selecting the optimal GS USP support system for your application, try our selector tool or visit their respective web pages GS USP-D and GS USP-R.

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