Tank farm transfer and cooling water intake duties share one constraint a horizontal pump cannot solve cheaply: the liquid level the pump must draw from sits well below grade, and NPSH available at a horizontal suction nozzle would be too low to trust across the full operating range. The fix, codified in API 610 as the vertically suspended (VS) pump family, is to put the impellers down at the liquid instead of trying to lift the liquid up to the pump. This article covers how the VS sub-types differ, which ones actually get specified for tank farms and cooling water intakes, and the intake-design and testing details that decide whether the installed pump performs the way the datasheet promised.
Why vertical instead of horizontal
A vertically suspended pump hangs its impellers, bowls or first stage down inside a wet pit, sump, tank or can, below the liquid surface at grade, with only the driver and discharge head sitting above ground. Because the impeller is already submerged, the pump gains the static head of the liquid column above it instead of having to lift liquid up to a suction nozzle before it can even begin developing head. That is why VS pumps are the default choice for tank farm transfer from low-lying storage, and for cooling water intakes drawing from a river, sea, or reservoir intake structure where the available water level can vary and NPSH margin at grade would otherwise be marginal or negative. The trade-off is civil cost and access: a horizontal pump in a pump house is simpler to inspect and maintain than a unit with its wet end sitting three, six or more metres below a removable deck, so the vertical configuration is usually chosen because the NPSH problem leaves no practical alternative, not because it is the cheaper or easier option to own. Variable frequency drives are common on cooling water and tank transfer duties too, since a VS pump running continuously often needs to track a varying flow demand rather than run fixed-speed against a throttled discharge valve, and buyers should confirm the motor and any downstream instrumentation are specified for variable-speed duty from the outset rather than added as an afterthought once the pump is already on order.
The API 610 VS sub-types
API 610 groups vertically suspended pumps into single-casing and double-casing designs, each with several configurations depending on the impeller type and how the discharge column is arranged:
| Type | Casing | Configuration | Typical use |
|---|---|---|---|
| VS1 | Single | Wet-pit diffuser pump, discharge through the column | Deep sumps and pits, tank farm transfer |
| VS2 | Single | Wet-pit volute pump, discharge through the column | Moderate flow/head sump service |
| VS3 | Single | Axial-flow impeller, discharge through the column | High flow, low head — large water transfer |
| VS4 | Single | Line-shaft sump pump, separate discharge column | Deep installations needing multiple lineshaft bearings |
| VS5 | Single | Cantilever sump pump, unsupported shaft, no submerged bearings | Sumps with dirty or abrasive liquid where a submerged bearing is undesirable |
| VS6 | Double (can) | Multistage diffuser pump inside an outer barrel | Cooling water intakes and tank farms where a deep civil sump is impractical |
| VS7 | Double (can) | Single-stage volute pump inside an outer barrel | Similar to VS6, lower head, single-stage duty |
VS6 and VS7 “can” pumps are configurations that may be specified specifically as a vertical turbine pump in tank farm and refinery service, because the outer barrel supplies the submergence the impellers need without a deep concrete sump — the can itself is simply set into a shallower excavation or mounted at grade. VS1 through VS5 remain common for water and firewater duties where a concrete wet pit or sump is being built anyway as part of the site civil works, so the extra cost of a can is not justified.
Tank farm transfer service
In a tank farm, vertical turbine pumps are typically used for product transfer between tanks, loading to pipeline or truck/rail racks, and firewater duty from a dedicated storage tank. The can (VS6/VS7) arrangement is common here because it avoids excavating a deep sump next to a tank foundation, and because the barrel can be sized to give the required submergence even when the source tank is drawn down close to its low working level. Buyers should confirm NPSH available is checked against the lowest operating level in the source tank, not just the normal level, since that is the condition that actually governs cavitation risk. On multi-product tank farms it is also worth confirming the wetted materials proposed suit the full range of products the pump may see over its life, not only the product it is being commissioned on, since a can pump is considerably more disruptive to pull for a materials change than a horizontal end-suction pump would be.
Cooling water intake service
Cooling water intakes draw continuously from a river, sea, canal or reservoir, often through a screened intake structure, and typically run 24/7 with high reliability expectations since a trip can force a process or power unit offline. Vertical wet-pit or can pumps suit this duty because they tolerate a fluctuating source water level without re-priming, and because the pump station footprint can be kept compact relative to a horizontal split-case alternative pulling from the same structure. Materials selection matters more here than in clean tank service — bronze, duplex stainless or coated carbon steel impellers and wear rings are common choices depending on water chemistry, biofouling potential and whether the source is fresh, brackish or seawater. Seawater and brackish intakes in particular tend to call for higher-alloy trims and, in some cases, cathodic protection on the can or column, so it is worth stating the water source and salinity explicitly on the inquiry rather than leaving the manufacturer to assume “cooling water” means fresh water.
Screening is worth specifying explicitly as part of the same enquiry rather than leaving it to the civil contractor. Trash racks stop large debris before it reaches the pump, while traveling or fixed screens downstream of the racks catch finer material that would otherwise foul impeller passages or wear rings over time. Screen mesh size and approach velocity both feed directly into the intake design calculation in the next section, so it is worth settling them before the sump or can geometry is finalised rather than treating screening as a separate, later decision.
NPSH margin and intake design
A vertical turbine pump’s real-world performance depends as much on how the sump, pit or can is shaped as on the pump itself. Poor intake design — insufficient submergence, sharp approach angles, uneven bay spacing on multi-pump installations — creates air-entraining vortices and pre-swirl at the impeller eye, which shows up as reduced head, lower efficiency, and vibration that a factory test on a clean test loop will never reveal. ANSI/HI 9.8 (Rotodynamic Pumps for Pump Intake Design) sets out minimum submergence, bay geometry and anti-vortex device guidance for both suction piping and wet-pit installations, and is the reference most specifications point to when a physical or CFD model study is required for larger or more critical intakes.
- NPSH available checked at the lowest anticipated operating level, not the normal level
- Minimum submergence over the bell/suction confirmed against ANSI/HI 9.8, not just the pump vendor's rule of thumb
- Bay spacing and approach geometry reviewed for multi-pump installations to avoid one pump's wake disturbing its neighbour
- Trash racks or screens sized so intake velocity stays low enough to limit debris carry-over and vortex formation
- A physical or CFD model study specified where the intake is large, unusual in geometry, or feeding critical service
Column, lineshaft and bearing arrangement
Two lubrication arrangements cover most vertical turbine pumps. An open lineshaft design uses the pumped liquid itself to lubricate the lineshaft bearings running down inside the column — simple and economical, and the usual choice for clean water such as a dedicated firewater or clean cooling water source. Where the pumped liquid is dirty, corrosive, or simply unsuitable as a bearing lubricant, an enclosed lineshaft design carries the shaft inside a separate tube with its own oil or grease lubrication, fully isolated from the process fluid — the more common choice for product transfer duties in a tank farm. Buyers should confirm which arrangement a quote assumes, since it changes both price and maintenance interval, and is not always obvious from a short datasheet description alone. Lineshaft bearing spacing and material (typically rubber, bronze or a composite such as a fluoropolymer blend) should also be confirmed against the actual setting depth, since a column length quoted generically rather than against the real submergence and sump depth is a common source of a pump that arrives too short — or unnecessarily long — for the installation.
Maintenance and pull-out access
A detail that is easy to underweight at enquiry stage is how the pump actually gets pulled for maintenance once it is installed. A deep-set wet-pit VS1/VS2 pump may need the full column and bowl assembly lifted clear of the pit, which means confirming crane or hoist access and headroom above the pump before the civil design is finalised, not after. A can-mounted VS6/VS7 pump is generally easier in this respect, since the bowl assembly can be withdrawn from the can without disturbing the discharge piping, but it still needs a clear vertical lift path and enough laydown space at grade for the full column length. Buyers specifying a vertical pump for the first time sometimes size the pump correctly but overlook the maintenance envelope around it, which turns a routine bowl overhaul into a scaffolding and rigging exercise that was never budgeted for.
It is worth asking the manufacturer, at quotation stage, for the expected pull-out length and weight of the longest single section, and confirming that against what your site can actually lift and lay down. This is a simple question that a manufacturer or sourcing partner familiar with vertical turbine pumps will answer without hesitation, and one that a generic reseller working purely from a catalogue often cannot.
Factory testing a vertical pump
A VS pump cannot be fully performance-tested on a standard horizontal test loop, because its behaviour depends on how it sits in a sump or can. Manufacturers instead test vertical pumps in a vertical test sump or pit sized to reproduce realistic submergence, and for can-mounted VS6/VS7 designs the pump is often tested complete with a test can, or with the actual can where practical, as a combined “string test” rather than testing the bowl assembly alone. Hydraulic performance is still verified against ISO 9906 acceptance grades, but buyers should confirm at enquiry stage whether the quoted price includes a witnessed test, and whether that test will be run at the pump’s actual setting depth and column length rather than a shortened test configuration — a detail that is easy to overlook until the test report arrives with a footnote about it.
Sourcing a vertical turbine pump package for a tank farm or cooling water intake? Browse our pumps & rotating equipment category, or send us your pump datasheet. Oillinko will circulate it to potential manufacturers experienced with the specific VS configuration and intake conditions your project needs, and check the returned offers against submergence, NPSH margin, lubrication arrangement and test scope before they reach you.
Frequently asked questions
What is the difference between a VS1 and a VS6 vertical turbine pump?
Both are wet-pit designs suspended into a sump, tank or can, but VS1 is a single-casing diffuser pump with the discharge routed straight up through the column to a baseplate at grade, while VS6 is a double-casing ("can") diffuser pump, where an outer barrel contains the pumped fluid and allows the pump to be installed above grade with the required submergence built into the can itself rather than a deep civil sump.
Why do vertical turbine pumps need less NPSH margin than an equivalent horizontal pump?
The impellers sit down inside the sump, tank or can, below the liquid level at grade, so the pump benefits from the static head of that liquid column instead of having to lift the liquid up to a suction nozzle. This largely removes suction lift from the NPSH calculation, which is why vertical turbine designs are the standard choice wherever NPSH available at grade would otherwise be marginal.
Do vertical turbine pumps need a separate intake design study?
For any wet-pit or sump installation, yes — the pump manufacturer's performance guarantee assumes the flow arriving at the impeller eye is uniform and free of air-entraining vortices. ANSI/HI 9.8 covers intake design (sump dimensions, minimum submergence, bay spacing, screens and anti-vortex devices), and a physical or CFD model study is commonly required for larger or more critical installations.
Is the shaft on a vertical turbine pump lubricated by the pumped product or separately?
Both arrangements exist. An open lineshaft design uses the pumped liquid itself to lubricate the lineshaft bearings, which is simple and common in clean water service. Where the product is not suitable as a lubricant — for example if it is dirty, corrosive or must not be diluted — an enclosed lineshaft design carries the shaft inside a separate tube with its own oil or grease lubrication, isolated from the pumped fluid.
