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Understand multi-stage pumps and sealing options: Part 1

02.01.2012  |  Gooch, L.,  AESSEAL plc, Rotherham, UK

Service life and cost impact what seals to use on your heavy-duty pump

Keywords: [pumps] [seals] [impeller] [centrifugal pumps] [mechanical seal] [API Plan 31] [API 682] [dual seals]

Whenever heavy-duty pumping is involved, produced-water injection (PWI) pumps are among those that come to mind first. Except for large piston pumps occasionally used for PWI duties, multi-stage centrifugal pumps are now primarily used in this severe service. Two typical styles of multi-stage centrifugal pumps—barrel and horizontally split—are shown in Figs. 1 and 2. PWI pumps are offered by different manufacturers, and this article will make no attempt to describe them all. Instead, the author will provide insight into sealing options for major pumps.

 

  Fig. 1.  Barrel-style multi-stage PWI pump.
  Source: Sulzer CPH barrel pump. 


 

  Fig. 2.  Horizontally split multi-stage pump.
  Source: David Brown Type DB34 horizontal
  split case pump.



Similarities exist among pumps.

There are obvious similarities within a particular style, i.e., barrel or horizontally split. In many ways, the internals of both styles are identical. Either style of pump can produce over 400 bar discharge pressure, although discharge pressures for barrel style duties are more typically in the 200 bar to 250 bar range. In both Figs. 1 and 2, the discharge nozzle is located near the center of the pump. This would indicate that several of the impellers making up the pump rotors are oriented for inlet flow originating near the drive-end. The other impellers are oriented in mirror-image fashion near the non-drive end (NDE) of the rotor. The design intent is to achieve hydraulic balance and to minimize thrust-bearing loads.

In Fig. 3, the cutaway shows a six-stage pump with fluid entering the pump at the nozzle (A). The fluid is fed through the first three impellers toward the center of the pump (B). Once the fluid exits the third impeller, it is redirected via a cross-over (C) to the outside impeller at the opposite end of the pump. From here, the fluid is pumped through the fifth and sixth impellers toward the pump discharge nozzle near (D).

 

  Fig. 3.  A cutaway view of a six-stage pump;
  three of its six impellers are oriented in mirror
  image fashion so as to achieve hydraulic
  thrust balance. Source: Goulds model 3600.


This design feature and the use of a balance line between the seal chambers means that the seals effectively operate at the pump suction pressure. Also, this type of pump is found in many other medium- and high-pressure (HP) applications.

Auxiliary or small-bore piping.

Auxiliary piping encompasses casing drains, vents, external lubrication and sealing lines; they are needed to complete the installation. The many possible seal-flush arrangements are described in vendor literature and denoted by API flush plan designations.1,2 The most important design features are:

• Casing drain line. This pipe is normally secured to the lowermost part of the pump casing and terminates in a block valve. The piping downstream from the valve generally leads to a common drain header or other secure disposal location.

 API Plan 31. Elevated-pressure fluid is bypassed from one of the pump stages ahead (upstream) of the last stage. The pressure of this bypass stream is then reduced by routing it through a set of orifice plates before it is piped to a cyclone separator. Clean fluid is taken off the top of the cyclone separator and fed into the seal housing. Dirty fluid is drawn off the bottom and fed back to pump suction.

Large pumps are often furnished with stand-alone lubricating oil consoles. If dual seals are used, then there could also be a Plan 54 unit in addition to Plan 31 flush piping.1,2 Even complex-looking stand-alone piping systems are straightforward if the review starts at the source. The reviewer looks for a lube-oil reservoir and a feed pump followed by filters, heat exchangers, pressure control valves and pressurized lubricant destinations. In essence, one traces each pipe and understands its purpose.

PWI installation layout and operation.

To obtain equal pressures around the underground crude oil reservoir, PWI systems have their own injection wells around the perimeter of the field. The flow into these injection wells can be adjusted by well operators to suit demand. Injection wells are then connected to an HP-ring main surrounding the field.

Except for temporary injection wells, there are two principal PWI layouts. One layout locates pairs of pumps in remote pumping stations roughly equidistant around the field. An example of this is the Dukhan field in Qatar, which has 11 PWI pumping stations with two pumps at each site feeding into a common ring main.

An alternative approach uses a central PWI pumping station, as shown in Fig. 4. The station in this illustration has more than 20 barrel and horizontally split pumps feeding into a central ring main. The top of the bearing lubrication console, plus the feed and return lines to the NDE bearing, are clearly visible in the foreground.

 

  Fig. 4.  A central PWI pumping station in Qatar.



Mechanical-seal pressure ratings.

Again, recall that the pump design is such that the seals are exposed to close to suction pressures. While the majority of pumps operate with suction pressures between 15 bar and 25 bar, there are instances where the stipulated suction pressure is quoted at 80 bar. Of course, both pump user-operator and manufacturer must cooperate to establish the actual operating conditions; 80 bar should be questioned.

The subject of seal-chamber rating vs. discharge-pressure rating for seals seems to cause confusion. Shell Oil Company’s engineering guidelines state that seals should be rated for the discharge pressure of the pump. It is normal, then, to find a 15 bar actual pressure in a seal chamber, although the seal is selected and designed for 200 bar. We are encouraged to be governed by API-682, which clearly states that HP-rated seals should not be used in applications where they are actually operating at much lower pressures. The framers of API-682 realize that HP seals prove problematic if they are continually subjected to much lower operating pressures. Although not mentioned in the API standard, HP seals and their support systems tend to be very expensive.

The underlying reasons as to why such seals may have been selected in the past are of interest. Seals were sometimes rated for discharge pressure because multiple pumps made up the process loop. If pumps feed into a common header system, then there exists the remote possibility of the discharge of one pump pressurizing another pump in the loop.

However, when the pumps are connected to the common header or ring main, reliability-focused users generally install a check valve—also known as a non-return valve (NRV)—downstream of the pump discharge nozzle. Figs. 5 and 6 show NRV installations in the discharge lines close to and/or adjacent to the pump discharge nozzle.

 

  Fig. 5.  An NRV installation in the discharge
  line of a horizontally-split pump.


 

  Fig. 6.  An NRV in the discharge line
  of a barrel-style pump.


It is generally known that older NRV designs had the potential for the swing plate to get dislodged, in which case non-running pumps could be pressurized by running pumps. This led to specification requirements that all components associated with such pumps be rated to the discharge pressure of the pump. Unfortunately, this has often led to short seal life and extremely expensive seals. The majority of oil companies are now using more reliable NRVs. This allows them to use API 682-compliant mechanical seals designed to operate at lower pressures.

We should always remind ourselves that it is better to address the causes of problems instead of wasting effort treating their symptoms. This is the strategy applied here by reliability-focused engineering contractors and users that select reliable NRVs and best-available sealing technology.

Sealing produced water.

Sealing of produced water and fluids with high salt contents is quite straightforward if we do not overlook basic principles. Of course, all mechanical seals run on a fluid film, as shown in Fig. 7. Also present is heat generated by friction and solids (or the potential for solids formation). Longer seal life is achieved as long as a stable fluid film separates the seal faces.

 

  Fig. 7.  Mechanical seals need a fluid film
  to separate the faces.


Seawater is probably the best quality water found at some PWI sites; however, land-based PWI systems use water drawn from underground aquifers, or they re-inject produced water separated from the extracted oil. This produced water or aquifer water is not generally highly abrasive, but it is full of is dissolved solids or salts. Salt crystals can be seen around the splash guard and the front support, as shown in Fig. 8. These salt deposits accumulate due to continual slight leakage from the seal.

 

  Fig. 8.  Seawater leakage caused crystals to form.


PWI pumps operate with relatively high pressures and have fairly large diameters. They generate a measure of heat that must be removed. Produced water has a high dissolved salt content, which, upon evaporation, reverts back to its crystalline solid phase. Heat removal and crystal formation must be well-controlled. Both will affect seal performance.

Neither fresh water nor continuous flushing or quench systems are practical options. However, two self-contained sealing arrangements are available and the method chosen depends on customer preference. Option 1 is a single seal—which is the lowest cost option, but it will only provide a maximum two-year service life with potable water. Option 2 is a dual-seal arrangement, which should reliably provide over three to five years of service if correctly maintained. Of course, the dual seal system is considerably more expensive. Each option will be discussed in greater detail in Part 2. HP

LITERATURE CITED

1 “Flush Plan Booklet,” AESSEAL Inc., Training Literature.
2 API-682, American Petroleum Institute.

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 role for AESSEAL and specializes in the upstream sector of oil and gas industry. Before joining AESSEAL, he worked for Fisher Rosemount in the control valve division and Mono Pumps where he served a mechanical technicians apprenticeship and went on to hold a project applications engineer’s position in UK sales. 




Have your say
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    All fields are compulsory.

nadhum hikmat
02.02.2014

dear sir .
It will be very much helpful ,if I could know how to calculate the thrust balance line in some multistage pumps ,my regards

Nattapol Ta.
09.09.2013

Have you ever faced about fine sand in produce water?
It always breakdown my mechanical seal even we used the API53B plan.
I am trying to find the solution for this case.
PS: Our sand from wellhead, 5-10 micron.

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