Final bore diameter and submersible pump passage

Start from the pump, not from the bit

Many borewells are drilled with whatever bit the rig has on the truck, and the pump is chosen afterwards. That order causes problems. A pump that fits on paper may not pass a tight casing joint, a slight bend in the hole or a section where the bore has closed in. Pulling a stuck pump, or redrilling to enlarge the hole, costs far more than planning the diameter correctly.

The better order is: decide the yield and head the well must deliver, choose a likely pump class, add the motor, cable, cable guard and any shroud, decide the casing and its inside diameter, and only then confirm the bit sizes. The compressor duty follows from those bit sizes and the depth.

Nominal bit size versus the space you actually have

A drilling log that says “6.5-inch bit” tells you the tool size, not the passage available for a pump. Several factors reduce the usable space:

• Casing inside diameter: the pump must pass through the casing, so its inside diameter, not its nominal size, is the first limit.
• Joints and couplings: socket joints, welded collars or reducers can be tighter than the pipe body.
• Bit wear: a worn bit drills an undersize hole, especially in abrasive granite. Gauge loss over a long hole is common.
• Deviation and alignment: a hole that wanders or doglegs reduces the effective straight passage for a long pump-motor assembly.
• Formation closure or ledges: clay that swells or fractured zones can narrow the hole after drilling.
• Cable and fittings: the flat or round cable, cable clamps and guards ride alongside the pump and take up space on one side.

The completion record should therefore show the as-built diameter at each depth, how it was established and where it is uncertain.

Typical bore and pump sizes in Indian borewells

Submersible borewell pumps are commonly sold by nominal bore class, such as 4-inch, 5-inch, 6-inch and larger. The class indicates the minimum bore or casing inside diameter the pump is designed for, and the manufacturer’s data gives the exact maximum outside diameter of the pump with its cable guard. The table below shows common pairings as a planning starting point only; always confirm against the actual pump, motor and casing data.

In much of Telangana and Andhra Pradesh, a common pattern is a larger top hole through the overburden with casing, then a 6.5-inch hole in hard rock. Whether the pump hangs inside the casing or in the open rock section below it changes which diameter is the real limit.

Clearance: what to check with the pump supplier

There is no single clearance that suits every well. The pump supplier should confirm it from the actual assembly and well geometry. Provide them with:

1. the pump and motor outside diameters, including the cable guard and any cooling shroud;
2. the total assembly length, which matters in a deviated hole;
3. cable size and how it will be clamped to the rising main;
4. the casing inside diameter, joint type and the narrowest known point;
5. the setting depth relative to casing and open hole;
6. the as-built drilling record, including bit changes and any problem zones.

Clearance also matters for motor cooling. Many submersible motors rely on water flowing past them at a minimum velocity; a motor in a very large bore with water entering from above may need a shroud. That is a pump-supplier decision, but it is another reason to record the real diameters.

How bore diameter changes compressor air needs

Once the bit sizes are fixed, the compressor must supply enough air to drive the hammer and lift the cuttings. Hole cleaning depends on the upward speed of the return air in the annulus between the drill pipe and the hole wall. In simple terms, flow equals annular area multiplied by velocity, and annular area grows with the square of the hole diameter.

The table below shows the free air flow needed to give an annular velocity of about 15 m/s (roughly 3,000 ft/min), a commonly used guide for dry air drilling, for typical bit and pipe combinations. These are illustrative calculations, not hammer requirements; the hammer’s own air consumption at working pressure must also be met, and wet holes or large cuttings need more.

The lesson is clear: moving from a 6.5-inch to an 8-inch hole roughly doubles the air needed for cleaning, and a 10-inch hole needs well over three times as much. A compressor that drills a 6.5-inch hole comfortably can struggle badly when the hole is enlarged for a bigger pump, even with the same hammer pressure.

Pressure follows depth and water, flow follows diameter

Diameter mainly drives flow. Depth and water inflow mainly drive pressure: a deep hole with a high standing water column needs more pressure at the hammer to keep it working and to clear the hole. That is why borewell compressors come in bands, typically 200–250 psig (13.8–17.2 bar) at 600–900 cfm for shallower work, 250–300 psig (17.2–20.7 bar) at 800–1,200 cfm for medium depths, and 300–400 psig (20.7–27.6 bar) at 1,000–1,500 cfm for deep, wet holes, with special units up to about 35 bar (507.6 psig).

When you plan a larger final diameter for a bigger pump, check both: enough flow for the larger annulus, and enough pressure for the depth. Otherwise the cost saved on a smaller compressor is lost in slow drilling, regrinding bits and redrilling.

Plan for installing and removing the pump

A pump is not installed once and forgotten. It will be pulled for repair, replaced with a different size as the water level changes, or inspected. Before ordering, ask how the pump will be lowered and lifted, whether any completion detail such as a reducer, a screen section or a tight joint could catch it, and whether the hole could be cleaned or redeveloped later through the same passage. These questions are easiest to settle before drilling the final section.

Geometry is only half the pump decision

Passing through the well is one requirement; delivering the required flow at the required head is another. A pump that fits physically can still be wrong for the yield, and a good pump curve does not guarantee it fits. Keep the yield test record, selected pumping rate, water-level data and system head calculation with the as-built drawing, and settle any diameter doubt before the pump order is final.

How Reddy Compressors helps

For borewell rig owners we supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts.

Reddy Compressors sells, rents and services borewell and DTH compressors as an independent multi-brand specialist, covering machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. We are not an authorised dealer of any brand, and we do not supply submersible pumps; our role is to make sure the air package can drill the diameter your pump needs. We have tie-ups with all major and reputed brands.

• Duty check by diameter: tell us the bit sizes and depth for each section and we confirm whether your compressor, or the one you plan to buy or hire, has enough flow and pressure.
• Hire for larger holes: our own fleet of diesel portable and high-pressure compressors when a project needs more air than your rig’s machine provides.
• New and used supply: brand-new compressors for contractors and long-term projects, plus used and reconditioned options where suitable.
• Service and spares: service across Telangana and Andhra Pradesh from Tarnaka, Secunderabad, and parts supplied across India, identified from the nameplate.

Call +91 99590 60718 or +91 92468 88812.

What to send us

• Planned pump class and setting depth, if known.
• Casing material, size and depth.
• Bit diameter for each section and drill pipe outside diameter.
• Target depth and expected water levels.
• Hammer size and current compressor make, model and rating.
• Site location and number of holes planned.

Frequently asked questions

What bore size do I need for a 6-inch submersible pump?

A 6-inch class pump is designed for a bore or casing with an inside diameter of at least about 6 inches, and in practice many drillers use a 6.5-inch to 8-inch hole in rock with suitable casing above. The exact figure depends on the pump’s outside diameter with its cable guard, the casing joints and how straight the hole is. Confirm with the pump supplier before drilling.

Can a 4-inch pump be used in a 6.5-inch borewell?

Yes, physically it will pass easily, and many wells use a smaller pump than the bore allows. Check with the pump supplier whether the motor gets enough cooling flow in the larger bore, as some installations need a shroud. Also make sure the smaller pump can deliver the yield and head the well and the application require.

How much air is needed to drill an 8-inch borewell?

For hole cleaning alone, an 8-inch hole with 4.5-inch drill pipe needs roughly 700 cfm to reach an annular velocity of about 15 m/s, around double a 6.5-inch hole. The hammer’s own air consumption at working pressure must also be met, and deep or wet holes need higher pressure. Many 8-inch jobs therefore use compressors from about 900 to 1,200 cfm or more.

Why does my pump get stuck when lowering into the borewell?

Common causes are a worn bit that drilled the lower hole undersize, a tight casing joint or reducer, a bend or dogleg in the hole, swelling clay or a ledge in fractured rock, or a cable guard that makes the assembly wider than expected. Check the as-built record and, if needed, have the hole reamed or surveyed before forcing the pump down.

Does a bigger bore diameter give more water?

Not by much in most hard-rock wells. Yield depends mainly on the fractures and aquifer the hole intersects, not on diameter. A larger diameter mainly allows a bigger pump and adds storage. It also costs more to drill and needs much more compressor air, so choose the diameter for the pump you need rather than in the hope of extra yield.

Should the bit size be checked during drilling?

Yes. Measure the bit gauge before starting and at intervals, especially in abrasive granite. A bit that loses gauge drills an undersize hole that may not pass the planned pump. Record bit changes and gauge readings on the drilling log so the as-built diameter of each section is known when the pump is selected.

Related pages

• Borewell and DTH compressor library
• Bore diameter vs air requirement
• Casing and unstable formation planning
• DTH hammer and bit selection
• New borewell compressor sales
• Borewell compressor rentals and hire