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Understand multi-stage pumps and sealing options: Part 2

04.01.2012  |  Gooch, L.,  AESSEAL plc, Rotherham, UK

As more processes involve high-pressure, toxic, flammable, lethal or explosive pumping services, better knowledge of rotating equipment options can boost productivity.

Keywords: [pumps] [seals] [dual seal] [API Plan 31] [single seal] [double seal] [flush point] [API 682] [metals]

(Editor's note: To read part 1 of this series, click here.)

Well-engineered single and dual seals are needed in the hydrocarbon processing industry (HPI). As more processes involve high-pressure (HP), toxic, flammable, lethal or explosive pumping services, a thorough understanding of the available options for rotating equipment, especially pumps, is mandatory. Seals in produced-water injection (PWI) services are typical applications deserving further investigation.

Option 1: Single seals for multi-stage pumps.

Fortunately, single seals are often a possible option for multi-stage pumps. Unlike dual-mechanical seals, single seals will not require a buffer fluid support system; thus, single seals are less expensive. Although the service life for single seals is about two years, these seals require more frequent replacement; some may last only a few weeks. The main issue with some single seals is often design-related. Some single seals ignore the deleterious effects of salt and other contaminants. It appears that careless selection routines allow API seals designed for clean-duty applications to be applied to dirty salt water.

A common seal style used in PWI service is shown in Fig. 1. The principal drawback of this design is a lack of clearance under the seal faces (point A). Single seals operating in a fluid with a high salt content often allow salt crystals to accumulate under the seal faces. The lack of clearance then causes the seal faces to hang up and fail. Moreover, these seals can sometimes experience problems if hard plating is used under the elastomer at point B. The plating tends to lift off unless the underlying substrate is corrosion resistant.

  Fig. 1.   A common style of mechanical seal
  often found in PWI water applications.
  As salt accumulates near A, fretting damage
  often occurs near B.

Another version of a single seal is shown in Fig. 2; it, too, has distinct drawbacks. The seal in question is a stationary cartridge seal, i.e., the spring-loaded face does not rotate—-a generally advantageous feature. However, multiple small springs are located in the contaminant-laden process fluid. This design should be considered less reliable than those that place the spring (or springs) away from the process fluid.

  Fig. 2.   Multiple springs are exposed to the
  process fluid in this seal.

An area of concern common to all seal styles is the location of the flush port location, as shown in Figs. 1 and 2. Unfortunately, the flush ports are directly placed over the seal faces. Most PWI pumps are fitted with API Flush Plan 31. This plan involves using cyclone separators. Apart from being expensive, cyclones are typically only about 97% effective in removing abrasive particles from the flush stream. When they are slightly undersized or are starting to clog, the effectiveness of cyclones is reduced even further. Solids then manage to reach the seal region and cause erosion damage. This is why the pump and seal specifications of at least one major international oil company have disallowed cyclone separators for several decades. This company has discovered a far more suitable alternative to flush arrangements for its PWI pumps. They are collectively called recent, or modern, seals.

Fig. 3 shows diagrams of the older and more recent seal designs. In the older design, debris may impede the axial movement of the rotating seal face. Also, the flush port is very close to the two seal faces. In the newer design, care is taken to move the springs away from both pumpage and flush fluid. The flush port is relatively far from the seal faces. Note: The older seal is conventional inasmuch as the axially moving seal face is part of the rotating assembly. In the modern design (Fig. 3), the nonrotating (“stationary”) seal can move axially.

  Fig. 3.   Examples of the older and newer styles of
  mechanical seals. (Source: AESSEAL Inc., Rotherham,
  UK, and Rockford, Tennessee.)


Single-seal options.

By addressing the key causes of premature failure, thoughtfully engineered, reliable single-seal solutions are available for PWI systems. The same principles can be applied to crude-oil transfer pumps, wastewater pumps, water outfall booster pumps and many others. When dealing with crude oil, consideration must be given to the presence of hydrogen sulfide (H2S). Even small amounts (approximately 10 ppm) can cause sulfide-stress corrosion in “conventional” metallurgies. So, the proper metallurgy must be selected. Recall that in H2S-containing services, the elastomers should be changed from the more commonly used Viton to Kalrez. Fig. 4 is a representative example of the single-seal alternative in PWI or related services. These seals are installed in pumps, as shown in Fig. 5.

  Fig. 4.   Cross-sectional view of a
  modern “stationary” single-seal option that
  does not allow process fluid to reach the
  small springs. Potential leakage flow would
  be seen exiting from the seal drain port.
  (Source: AESSEAL Inc., Rotherham, UK,
  and Rockford, Tennessee.)

  Fig. 5.  Two single seals are installed in two pumps.
  (Source: AESSEAL Inc., Rotherham, UK,
  and Rockford, Tennessee.)


Option 2: Dual-seal option.

Conventional industrial applications tend to use dual seals whenever difficult-to-seal fluids are involved. This thinking would also prevail in the case of fluids with high salt content. Dual seals offer extended service life because the fluid film is controlled and the salt-crystal accumulation is effectively prevented.

Yet, dual seals in PWI systems are often impractical because of pump location and geography. Much of the Middle East is considered an extreme environment for typical PWI stations. The average daytime temperature can exceed 45°C (115°F), and the nearest freshwater supply could be more than 40 miles away. As a rule, severe station environments make using dual seals problematic. The primary issues are quite obviously how to conserve fresh water and how to cool the barrier fluid that separates the inboard and outboard seals.

Whenever dual-seal systems are used in harsh environments, they are expensive. The costs escalate when the seals are rated for full-pump discharge pressure. Conversely and not surprisingly, dual-seal systems have significantly extended seal life.

Fig. 6 shows a particular HP dual seal found on PWI and crude-oil transfer pumps. Its manufacturer supplies seals rated to the full discharge pressure of the pump. The owner-user is instructed to operate with a barrier fluid pressure in excess of 100 bar, even if the seal environment does not exceed 15 bar.

  Fig. 6.  Side view of an HP seal offered
  by a prominent seal manufacturer.

The HP rating of certain seals can lead to unforeseen drawbacks. So as to prevent O-ring extrusion, the clearances between the component part tolerances must be extremely tight. Tight clearances in dirty fluids are prone to clogging and to elevated risk of seal-face hang-up.

Materials of construction.

Often single and dual seals use the same materials of construction. Since corrosion is an issue, the metallic components must be Hastelloy C, unless dictated and specified otherwise by the owner-user. Viton serves as the traditional elastomer; Kalrez is used if H2S is present. Silicon carbide/silicon carbide face combinations are used for single seals and for the inboard seal faces of dual seals. Silicon carbide/carbon combinations are used on external dual-seal faces. One successful approach to sealing produced water, as shown in Fig. 4, is giving due consideration to potential problem areas:

Best materials of construction include C 276/SiC/SiC/Viton or Kalrez. Using the correct materials of construction virtually eliminates corrosion issues.

Springs not contacting process fluid. Multiple small springs offer many benefits over a single, large-coil spring. However, small springs are prone to clogging. An advantageous design deliberately places the springs outside the process fluid. This may be considered a simple item. Yet, it is often overlooked, and not even API-682 makes reference to the issue.

Large clearance under the seal faces. Comparing seal cross-sectional views from different manufacturers will reveal how the properly designed modern seals have greater clearance under the seal faces than seals potentially offered by another manufacturer. Suitable designs consider that the fluid has a high salt content and will crystallize under the seal faces. There should be sufficient room for this to happen without restricting seal-face movement.

Directed-flush port. For applications where solids could potentially cause a problem or where the customer wishes to move away from cyclone separators, at least one major manufacturer offers a directed flush design. This design allows solids to be directed away from the seal faces while still providing circulation in the seal chamber.

Modern dual-seal options.

With oil companies moving toward lower-pressure-rated seals and striving for longer equipment operating times, users are compelled to find knowledgeable seal manufacturers and suppliers. Compliance with the dual-seal recommendations of API-682 is highly desirable as well.

Apart from being modular in design and thus allowing for interchangeability between single and dual components, the modern O-ring pusher dual seal, as shown in Fig. 7, has many advantages over traditional seals. It represents a true dual seal with two independently mounted seal faces. Both seal faces are internally pressure-balanced. The inboard seal faces are double-balanced and all faces are flexibly mounted.

  Fig. 7.  Side view of a modern dual
  mechanical seal. (Source: AESSEAL Inc.,
  Rotherham, UK, and Rockford, Tennessee.)

A standard dual seal is typically used in conjunction with a conventional thermosiphon system in duties or at sites where cooling water is readily available. At such locations, most PWI and water-disposal pumps use dual seals per API Plan 54 systems, as shown in Fig. 8. The modern dual seal is then often supplied with the pumping ring removed (Fig. 7). It achieves a measure of enhanced cooling between the seals while retaining all the advantages of interchange with other seals onsite.

  Fig. 8.  An API Plan 54 cooling unit.
  (Source: AESSEAL Inc., Rotherham, UK,
  and Rockford, Tennessee.)

There is, however, a note of caution. When fitting seals to PWI or HP water-disposal pumps, be sure to use nickel-plated, carbon-steel grub screws. These must be secured to the shaft by dimpling the shaft surface to rule out seal sleeve slippage during operation.

More on Plan 54 systems.

As mentioned earlier, heat removal from the seal is a prime concern, especially so that the pumps can operate in high ambient conditions. With PWI stations generally situated in remote locations, cooling units must be self-contained. Figs. 8 and 9 are two examples of API Plan 54 units.

The Plan 54 water-circulating cooling unit in Fig. 9 is perfectly acceptable at locations with ample cooling-water supplies. Conversely, air cooling (Fig. 10) is the preferred method in regions or areas where water is at a premium or not available. An air-cooled Plan 54 unit has the standard water-cooled shell-and-tube heat exchangers replaced with air fans or blowers. A second example is illustrated in Fig. 11.

  Fig. 9.   A water-circulating
  API Plan 54 cooling unit
  operating onsite. (Source:
  AESSEAL Inc., Rotherham,
  UK, and Rockford, Tennessee.)

  Fig. 10.   A high-capacity air
  cooler unit for an API Plan 54
  seal support system. (Source:
  AESSEAL Inc., Rotherham,
  UK, and Rockford, Tennessee.)

  Fig. 11.    An air blower unit
  for an API Plan 54 seal
  support system.

The systems used in the oil and gas industry are generally far more sophisticated than those found at normal industrial sites. They are also more expensive and often equal (if not exceed) the value of the seals involved. It is, therefore, vital that the reliability-focused user-purchaser gives equal attention to seals and seal-support systems.

Comments.

The intent of this two-part article is to give the reader a basic insight into the many opportunities for well-engineered components offered by highly competent seal manufacturers. Most of the applications illustrated are either dirty water or dirty oil. There obviously are a multitude of applications that can greatly benefit from best available technology. HP

End of series: Part 1, February 2012

The author 

 
  Lee Gooch has been with AESSEAL for 14 years. He has held various positions within the company including project engineer and senior sales engineer. He now is responsible for business development and applications engineering roles for AESSEAL and specializes in the upstream sector of the oil and gas industry. Before joining AESSEAL, he worked for Fisher Rosemount in the control valve division, and for Mono Pumps where he served in a mechanical technicians apprenticeship and went on to hold a project application engineer’s position in UK Sales. 




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(not given)
04.26.2012

This document is misleading and inaccurate.
Springs are in the product because they need to be wet to prevent the scaling that occurs on the atmospheric side. Scaling often occurs in the dry side of the seal as the deposits are left when the pumped media flashes off in the seal gap, not in the process fluid. The "old" designs shown also have variants commonly available where the springs are out of the process, if there is a potential benefit. Not all of these seals have the springs in the product and to suggest otherwise is misleading. The same applies to large clearances under the faces. All seal companies can and do offer this. Also suggesting these seals are "considered less reliable" when MTBF on WI duties is sometimes >8 years is a ridiculous statement to make. AES can only dream of MTBF like this since they are essentially offering a "modern" (= unproven) product which is low cost and low performance. I would also suggest that the flush connection over the seal faces will remove the heat generated by the seal effectively, whilst the "modern" seal looks to be flushing the stuffing box. Not exactly helpful in reducing the heat generated or reducing the risk of scaling!
Ignoring the MAWP requirement of pressure containing parts such as the seal system is conflicting with API and downright dangerous. Would you trust an NRV with your life?....AES want you to. Rate the system properly for MAWP and if the worst does happen, you know that the safety of your people is secure. On the other hand since AES don't have a product than take MAWP, just pretend it isn't that important and hope no-one gets killed when the 50 bar system has 450 bar in it.
As for the grub screws and dimpling the shaft....? Oh dear....

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