Multistage pumps for light hydrocarbon (Part 1)

In The Hitchhikers Guide to the Galaxy, the single most important thing a traveller should know at any given moment is where their towel is.?The HHGTTG goes into great?detail about why this would be the case, but suffice it to say that being prepared for any eventuality figures highly.

Extending that metaphor to Engineering, the equivalent of a towel for a practicing Engineer is knowledge.?An informed Engineer is far more at ease, comfortable and able to deal better with the various corporate vagaries pitched his/her way on an often daily basis.?(This also leads into?first rule of my?Two Rules of Engineering, which I might?share on request at some future date).???

So with your towel grasped firmly, onto the topic at hand.?With the advent of large scale fracking in the USA, a lot of light hydrocarbons (HC) became available at a competitive cost?for use as feedstock in chemical processes.?This in turn has lead to large increase in demand for pumps to handle these fluids.?(For the purposes of my post I'll define light HCs as those with a Specific Gravity (SG) below 0.7).?Many of the services require high pressures which in turn requires multistage pumps.?These have specific design and application challenges which I plan to cover in this series of posts.?

This?series is an abridged version of a presentation I recently gave to several engineering contractors in Houston.?If your company is interested in the topic, feel free to contact me to discuss whether we could provide a session for your engineers.

This first post will cover the pump types normally used and their materials of construction.?The?three main pumps types used for API 610 compliant service are listed below:

• BB3 axially split multistage, between-bearings
• BB5 double-casing radially split multistage, between-bearings
• VS6 double-casing diffuser, vertical suspended??

You will notice I left out the BB4 ring section pump.?You sometimes see these deployed on non critical services.?However they cannot be made API 610 compliant and hence I've excluded them.

API610??- BB3 axially split multistage, between-bearings

• ?Temperature to 400°F (205°C)
• Pressure to 4250 psi (293 Bar)
• Specific Gravity (SG) down to ≈ 0.3 (limit based on gasket permeability)

This multistage pump style is the workhorse of the pipeline industry.?It is preferred due to the ease of field servicing (take off the top half casing and the rotor can be inspected/removed in one piece and a new one swapped in.?Being a single casing, it has a lower first cost than double-casing designs such as BB5 or VS6.?Impellers are normally mounted back to back reducing axial thrust and improving rotor stability.?

API 610 recommended limits for this style of pump are

• Fluid SG ≥?0.7
• MAWP ≤?1450 psi (100 bar)

These limits are routinely ignored by many customers, to the extent that they are rendered fairly worthless. (and really should be revisited by the API 610 committee in a future update to the standard).?

The main limiting factor for very low SG hydrocarbon fluids is permeation through the gasket (which results in increased HC emissions).?Certain gasket?material compositions can reduce this somewhat but not prevent it.?The exact SG limit on the application of this pump type depends who you ask, but ITT Goulds Pumps has a recommended process that I'll talk about in Part 2.

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API610??- BB5 double-casing radially split double casing multistage, between-bearings

• ?Temperature to >800°F (425°C)
• Pressure to >4250 psi (293 Bar)
• Specific Gravity (SG)?down to ≈ 0.15 (limit dependent on rotordynamics)

This multistage pump style is utilized when pump reliability and minimum emissions are critical success factors.?The?barrel casing is usually sealed with?spiral wound gaskets, which reduces gasket permeation to a practical minimum.?The?inclusion of a barrel?casing increases cost and these will typically have a noticeably higher first cost than the BB3 and BB4 pump types.

Most BB5 designs available in the marketplace are?inline?diffuser designs, which results in higher axial thrust than a comparable BB3.?This may require?higher capacity?bearing arrangements, such as forced lubrication tilting pad thrust bearings.

The limiting?SG?with this design?usually ends up being determined by the rotordynamics.?All multistage machines in the marketplace today rely on the Lomakin Effect for stable operation.?A lower SG means less Lomakin?Effect and at some point, there isn't enough to?achieve acceptable operation.?More on this topic in Part 2.???

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API 610 - VS6?double casing diffuser,?vertical suspended??

• Temperature to 400°F (205°C)
• Pressure to 1480 psi (102 Bar)
• Specific Gravity (SG)?down to ≈ 0.15 (limit dependent on rotordynamics)

?This multistage pump style is often utilized when NPSHa is very low (or even zero in the case of a boiling fluid).?The principle is simple, a standard vertical turbine pump is mounted inside a suction can.?The length of the suction can (and the pump) are determined by how much NPSHa is available.?The lower the NPSHa, the longer the can needs to be to ensure sufficient NPSH is available at the pump inlet.

This pump solution has the advantage of requiring very little floor space.?It does however require excavation to accommodate the suction can.?Other limits are the size of vertical motor that can be used ≈ 2700 HP (2 MW) is about the upper practical power limit (exceptions exist of course).?Monitoring of the pump (and the rotor health in particular) is more difficult than with a horizontal pump and some end users impose limits on this style of pump for this reason.

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Materials of Pump Construction

The main driver here is the Minimum Design Metal Temperature (MDMT).?You need to consider the minimum process temperature but also fault conditions such as uncontrolled boil off of the fluid ( auto-refrigeration).??We had one recent example where the pump MDMT ended up being?below -155°F??(-104°C) due to the possibility of auto-refregeration, far below the normal process temperature.

For MDMT down to -50°F?(-46°C), the normal recommended material choice is S-6 which is a low temperature carbon steel casing with chrome steel internal parts.?While it is possible to go to lower temperatures with carbon steel, the testing becomes?very onerous, supply is limited and a material?impact?test failure will?severely disrupt?your project plan since the?affected part must be remade.?Thus ITT Goulds Pumps guidance is to limit S-6 construction to?ASTM A352 Gr LCB?and the temperature limit?noted.

For MDMTs?below -50°F??(-46°C), the normal recommended material choice is A-8 which is a?fully austenitic stainless?steel casing?and internal parts.?This material choice adds significant cost since the material is weaker than carbon steel (a heavier pump), more expensive and internal parts often require?costly overlays to prevent galling.

One final comment on the subject of materials - if your pumped fluid is clean (which means ≤ 300ppm solids), I strongly recommend utilizing non metallic wear parts.?API 610 Table H.3 details various acceptable choices.?The reasons I recommend them are:

• Light HCs have less stabilizing effect on the rotor – smaller clearances are required which can cause galling with metallic parts
• Light HCs have low lubricity – increasing metallic part wear in start/stop
• With low temperatures - achieving 50 Brinell hardness differences at wear parts is more difficult (weld overlays are often required increasing cost)

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Ok so that is it for this post.?In Part 2 we'll cover

• When it is appropriate to switch from an axially split pump (BB3) to a radially split?pump (BB5)
• Rotordynamics and how light HC affects the design
• NPSHa, NPSHr, fluid heating and balance line management

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In Part 3?we'll cover

• Pump testing in the factory
• Pump storage, installation and commissioning?

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Until then

Beatus Centrifuga