Rotary gear pumps for viscous, fuel and lube oil duty: selecting the right NBL-DEL type

A rotary gear pump is the right choice when the liquid is thick, lubricating or needs to be delivered at pressure regardless of how the system resists it. Picking the correct NBL-DEL gear pump comes down to six things: gear geometry, material of construction, viscosity at the operating temperature, pump speed, relief valve arrangement and motor power. New India Electricals Ltd (NIEL) supplies seven NBL-DEL types, from a 0.5 LPM gerotor lube pump to a 200 m3/hr herringbone pump for crude and furnace oil. Below, each type is mapped to its duty.

TL;DR

• Gear pumps are positive displacement machines. Flow follows speed, not system resistance, so every installation needs a pressure relief path.

• NBL-DEL Type HG and HGX suit lube oil, paints, edible oil, transformer oil and LDO in cast iron construction up to 10 and 12 kg/cm2.

• Type HGSX and RDMS are SS-316 pumps for food grade and corrosive viscous liquids; RDMS adds herringbone gears, sizes to 6 inch and steam or water jacketing.

• Type RDRN handles crude, dirty oil, HSD, kerosene and furnace oil up to 200 m3/hr, and the Fuel Injection gear pump delivers burner and boiler fuel at up to 35 kg/cm2.

• Viscosity sets the speed. NIEL brochures rate most types at 1440 rpm up to 200 or 500 cSt and call for lower speed above that, which cuts capacity in proportion.

• Motor power is duty pressure multiplied by unit power per kg/cm2, plus the no-load viscous power, plus a margin.

• Furnace oil must be heated before it is pumped; the Bureau of Energy Efficiency puts typical burner tip preheat at about 105 degrees C.

What makes a rotary gear pump different from a centrifugal pump?

A rotary gear pump traps a fixed volume of liquid between gear teeth and the casing and carries it from suction to discharge on every revolution. The KSB Centrifugal Pump Lexicon defines it as a rotating positive displacement pump in which the meshing teeth act as both displacement bodies and separating elements, which makes it self-priming and able to generate high static pressure.

In a centrifugal pump, flow falls as system resistance rises and the pump settles on its curve. In a gear pump, flow stays close to fixed for a given speed. The KSB lexicon entry on positive displacement pumps states it directly: “The head is independent of the pump speed.” Pressure rises until the pump delivers its volume, the motor stalls, a joint fails or a relief valve opens.

Within its rating, a gear pump also handles thick liquid better than a centrifugal pump, which loses head and efficiency as viscosity climbs. That is why lube oil systems, fuel oil trains, paint circuits and edible oil lines use gear pumps, and why NIEL lists its NBL-DEL range separately from its centrifugal pumps.

Which NBL-DEL gear pump type suits which duty?

NIEL lists seven NBL-DEL types. Cast iron Types HG, HGX and RDRN cover oils and fuels. Stainless steel Types HGSX and RDMS cover food grade and corrosive liquids. The Fuel Injection gear pump and FTMP gerotor are compact designs for burner firing and lubrication. The table below uses the figures published on each NIEL product page.

Start from the liquid, not the flow. If the liquid touches food, or attacks cast iron, the choice narrows to Type HGSX or Type RDMS. If it is a fuel for a burner at high pressure, the Fuel Injection gear pump is the purpose-built option. If it is lube oil on a machine with limited space, the FTMP mounts directly on a flange. For bulk oil transfer and general viscous duty, choose between Type HG, Type HGX and Type RDRN on capacity and pressure.

When should you choose external, internal or gerotor gear geometry?

External gear pumps suit general transfer and moderate pressure across the widest size range. Internal gear pumps suit high pressure fuel delivery in a compact, self-priming package. Gerotor pumps suit small, quiet, flange-mounted lubrication. In the NBL-DEL range, HG, HGX, HGSX, RDMS and RDRN are external; the Fuel Injection pump is internal; the FTMP is a trochoid gerotor.

Within external gear pumps, tooth form matters as size grows. The NBL-DEL hub page notes that large pumps use helical or herringbone gears for smoother flow. Type HG uses single helical gears. Types RDMS and RDRN use double helical, or herringbone, gears. NIEL’s RDMS page states the purpose: herringbone gears prevent axial thrust. A single helical gear produces thrust along the shaft under load, which the bushes and thrust faces must absorb. In a double helical gear the two opposing helices cancel each other, which is why the two NBL-DEL types that reach 6 inch both use this form.

The Fuel Injection gear pump uses an internal gear with a nitrided alloy steel rotor and idler, a mechanical seal and self-priming action. NIEL rates it at up to 35 kg/cm2, the highest pressure in the NBL-DEL range, and for clockwise or anticlockwise rotation. It is sized in litres per hour, from 60 to 2500 LPH, because burner firing needs a small, steady flow at high pressure rather than bulk transfer.

The FTMP uses a trochoid gerotor, an inner rotor turning inside an outer ring. NIEL describes it as compact and low noise, with a monoblock version available, which suits lubrication circuits where the pump mounts on the machine.

How does viscosity change the speed you should run a gear pump at?

Viscosity sets the maximum safe speed. NIEL’s NBL-DEL brochures rate most types at 1440 rpm up to a stated viscosity limit, then require lower speed above it. Lower speed reduces capacity in proportion, so a pump selected at 1440 rpm on thin oil may need to be one size larger once it runs slower on thick oil.

The published limits differ by type:

The reason is filling. Thick liquid moves slowly into the gear cavities on the suction side. At high speed, the teeth unmesh before the cavity fills, so the pump starves, capacity drops, and the collapsing voids cause noise and wear similar to cavitation. NIEL’s HGX brochure lists the benefits of running slower: better efficiency, lower noise, better NPSH performance and longer life. For the suction side check, see NIEL’s guide to NPSH and cavitation.

There are three practical ways to get the lower speed. Specify a 6-pole or 8-pole motor. Use a belt drive, gear reducer or pulley arrangement. Or use a variable frequency drive, which lets the operator trim speed as the liquid’s temperature and viscosity change through the day. NIEL supplies low voltage AC drives for this. When a pulley or gear drive is used on an RDMS pump, NIEL’s brochure calls for a bearing block so that the belt or gear load does not fall on the pump shaft and bushes.

Always state viscosity at the pumping temperature, including cold start, and size the speed for the worst case the pump will see.

How do you calculate motor power for an NBL-DEL gear pump?

NIEL’s HGX brochure gives a two-part method. Multiply the unit power per kg/cm2 of pressure for the chosen size by the duty pressure. Add the no-load viscous power for the liquid’s viscosity, published in columns for 200 and 500 cSt. Then add a margin before selecting the motor frame.

The two parts reflect two different loads. The pressure term is the hydraulic work of pushing a fixed volume against the system. The viscous term is the friction of shearing thick liquid in the gear mesh, the bushes and the clearances, which the pump consumes even at zero discharge pressure. On thin oil the viscous term is small. On thick oil it can dominate, which is why a pump that runs cool on lube oil can overload its motor on the same duty with a heavier grade.

NIEL’s Type HG brochure instead gives a motor sizing table at 1440 rpm for 1500 SSU. The HG and HGX brochures use different viscosity units, so convert before comparing.

Two further points apply to power. First, if the relief valve is set above duty pressure, size the motor for the relief setting, because the pump will reach that pressure whenever a downstream valve closes. Second, the HGSX brochure says small sizes can run at 1400 rpm up to 1000 cSt only with a higher motor rating. Speed and viscosity limits and motor power are linked; one cannot be changed without checking the others. For the motor itself, NIEL supplies closed cage type motors for general pump duty.

Why does every gear pump need a pressure relief valve?

Because a positive displacement pump keeps delivering its volume whatever the system does. If a discharge valve closes, a filter blocks or a line freezes solid, pressure rises until something yields. The KSB lexicon confirms that head in a positive displacement pump is independent of speed; the system, not the pump, sets the pressure.

NIEL builds the relief path into most of the range:

• Type HG: the brochure states “Pressure relief valve will be provided in all models.”

• Type RDRN: inbuilt relief valve.

• Fuel Injection gear pump: built-in relief valve, with internal or external bypass.

• Type RDMS: available as an add-on or built-in relief valve; specify which when you enquire.

• Types HGX, HGSX and FTMP: confirm the relief arrangement at enquiry.

An internal relief valve returns flow to suction inside the pump. It protects the pump and motor, but the recirculated liquid heats with every pass, so it is a safety device, not a flow control. An external bypass returns flow to the tank and suits duties where the pump may run against a closed burner valve for longer.

Set the relief valve above working pressure, below the pump’s rated maximum and below the weakest discharge component’s rating, then size the motor for that setting.

What changes when the duty is furnace oil or other heavy fuel?

Heavy fuel must be heated before it can be pumped, then heated again before it can be atomised. The Bureau of Energy Efficiency’s guide to fuels and combustion calls viscosity “the most important characteristic in the storage and use of fuel oil” and warns that furnace oil below 25 degrees C is not easily pumpable.

BEE describes two preheating stages. Bulk tank steam coils or an outflow heater bring the oil to a pumpable viscosity. A second heater brings it to burner tip viscosity, which BEE gives as about 100 Redwood seconds, requiring heating to about 105 degrees C. BEE’s property table gives pour points of 20 degrees C for furnace oil, 18 degrees C for LDO and 72 degrees C for LSHS, which needs heat even to leave the tank.

This splits a heavy fuel system into two pump duties. The transfer pump, from storage tank to day tank, handles warm but still viscous oil at modest pressure; Type RDRN is listed by NIEL for crude, furnace oil, HSD, LDO and dirty oil at up to 200 m3/hr. The burner feed pump handles hot oil at high pressure and low flow; the Fuel Injection gear pump is rated by NIEL to 200 degrees C, 500 cSt and 35 kg/cm2.

Two checks follow. Pumping temperature must stay within the pump’s rating, including seal and bush materials. And where the plant’s area classification requires it, the motor enclosure must match; NIEL supplies flame proof motors, but the classification itself must come from the plant’s own hazardous area study.

For liquids that solidify when they cool, such as bitumen, tar, molasses and some food products, a jacketed pump keeps the liquid mobile inside the casing. NIEL offers steam or water jacketing on Type RDMS for liquids that solidify.

When is stainless steel worth the extra cost over cast iron?

Choose SS-316 when the liquid is food grade, corrosive, or must not pick up iron contamination. Choose cast iron for mineral oils, fuels, paints and transformer oil, where the liquid itself protects the metal. In the NBL-DEL range, HGSX and RDMS are SS-316; HG, HGX, RDRN, Fuel Injection and FTMP are cast iron.

The difference goes beyond the casing. NIEL’s material specifications show how each design is built for its liquid:

Type HG’s oil seal and sintered bronze bushes rely on the pumped oil for lubrication. That suits lube oil and edible oil but not thin, corrosive or non-lubricating liquids. Type HGSX uses DU bushes and a Teflon Z-pack seal, which tolerate liquids such as glycol and glycerine and meet the hygiene demands of ghee and butter. Type RDMS runs DU bushes against a bronze wear plate.

Wood pulp, fruit pulp and dirty oil bring a third consideration: solids and abrasion. NIEL lists wood pulp and dirty oil for Type RDRN, and fruit pulp for Type RDMS. The RDRN comes in two bearing versions, needle roller (FTRN) and bronze bush (FTRB). Confirm which suits an abrasive duty when enquiring. For thin corrosive chemicals rather than viscous ones, a centrifugal stainless steel pump may be the better fit.

What should a gear pump specification sheet include?

Send these details with every NBL-DEL enquiry. Each missing item forces a guess on viscosity, speed or power.

• Liquid name, grade and whether it is food grade, corrosive, abrasive or contains solids

• Viscosity at pumping temperature, in cSt, with the unit stated, plus viscosity at cold start

• Pumping temperature range, minimum and maximum

• Required flow in LPM, LPH or m3/hr at the rated condition

• Discharge pressure at duty and maximum system pressure, in kg/cm2

• Suction condition: flooded or lift, suction line length and diameter, tank level

• Relief valve requirement: built-in, add-on, internal or external bypass, and set pressure

• Drive arrangement: direct coupled, monoblock, pulley, gear reducer or variable frequency drive

• Motor supply voltage, frequency and enclosure, and the area classification if the site has one

• Jacketing need for liquids that solidify, and heating medium (steam or water)

• Material preference for body, gears, bushes and seal, if the plant has a standard

• Duty cycle: continuous, intermittent or standby

  Frequently asked questions

  Which NBL-DEL gear pump is best for furnace oil?

  It depends on which part of the fuel system the pump serves. For storage to day tank transfer, NIEL lists Type RDRN, a cast iron double helical pump for crude, furnace oil, HSD and LDO at up to 200 m3/hr and 20 kg/cm2, with an inbuilt relief valve. For burner or boiler feed, the NBL-DEL Fuel Injection gear pump is rated for up to 500 cSt, 200 degrees C and 35 kg/cm2 at 60 to 2500 LPH. Both duties need the oil heated first; the Bureau of Energy Efficiency notes furnace oil below 25 degrees C is not easily pumpable.

  Can a gear pump run against a closed discharge valve?

  Only briefly, and only with a relief valve fitted. A gear pump keeps delivering its fixed volume whatever the downstream system does, so against a closed valve pressure rises until the relief valve opens, the motor overloads or a component fails. An internal relief valve returns flow to suction, but the recirculated liquid heats with every pass. Where discharge may stay closed for longer, as in a burner fuel train, an external bypass to tank is better. NIEL’s Fuel Injection gear pump offers both. Size the motor for the relief setting.

  Why does NIEL recommend reducing speed for high viscosity liquids?

  Because thick liquid cannot fill the gear cavities fast enough at full speed. On a heavy oil, the cavity is still partly empty when the teeth carry it round, so the pump under-delivers, runs noisily and wears faster. NIEL’s HGX brochure rates the pump at 1440 rpm up to 500 cSt and calls for lower speed above that on sizes over 1-1/4 inch, citing better efficiency, lower noise, better NPSH performance and longer life. Capacity falls in proportion to speed, so select a size that meets the flow at the reduced speed.

  What is the difference between NBL-DEL Type HGSX and Type RDMS?

  Both are SS-316 pumps for food grade and corrosive viscous liquids, but they differ in size, gear form and options. Type HGSX is a three-piece investment cast pump with a Teflon Z-pack seal, covering 8 to 300 LPM at up to 11 kg/cm2 in sizes to 2-1/2 inch. Type RDMS is a four-piece pump with herringbone gears that prevent axial thrust, covering 12 to 2000 LPM at up to 20 kg/cm2 in sizes to 6 inch, with optional steam or water jacketing. Choose HGSX for smaller flows and RDMS for larger flows, higher pressure or heated duty.

  Is a gerotor pump suitable for lube oil systems?

  Yes, for small flows. The NBL-DEL FTMP is a trochoid gerotor pump that NIEL lists for lubricating oil. It is compact, low noise and flange mounted, with a monoblock version available, and covers 0.5 to 40 LPM at up to 15 kg/cm2 in eight sizes from 1/4 to 3/4 inch BSP. The body is cast iron and the rotor sintered cast iron. For larger flows, such as a central lubrication system, Type HG covers up to 500 LPM and Type HGX up to 350 LPM.

  How do I size the motor for a gear pump on heavy oil?

  Add the pressure load and the viscous load, then a margin. Per NIEL’s HGX brochure, multiply the unit power per kg/cm2 for the pump size by the duty pressure, add the no-load viscous power published for 200 and 500 cSt, then apply a margin. The viscous term applies even at zero discharge pressure and can dominate on heavy oil. Size for the relief valve setting if it sits above duty pressure, and for cold start viscosity if the pump must start cold.

For help matching an NBL-DEL type to your liquid, viscosity and duty point, send the specification details above to the NIEL pumps team through the contact page. An engineer will confirm the type, size, speed and relief arrangement before quotation.
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