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
pumpsbarrel and horizontally splitare 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
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
Fig. 3. A cutaway view
of a six-stage pump;
three of its six impellers are oriented in
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
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
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
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
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
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
Companys 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 valvealso 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
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
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
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
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
sealwhich 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
1 Flush Plan Booklet, AESSEAL Inc.,
2 API-682, American Petroleum Institute.
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
engineers position in UK sales.