Deep Hole Drilling for the Liquefied Natural Gas Industry: LNG Pump Shaft Bores, Cryogenic Valve Stems, and Boil-Off Gas Compressor Piston Rods

A manufacturer of multistage LNG cryogenic pumps (304L SS shaft, 150 mm x 8 m, requiring 60 mm x 8 m centre bore) used BTA drilling with counter-rotational workpiece (20 rpm) and tool (400 rpm) at Vc = 45 m/min, f = 0.12 mm/rev, oil at 40 bar. Bore straightness 0.08 mm/m. Dynamically balanced to G2.5. Cryogenic tested at -196 C LN2 with zero measurable distortion.

LNG Component Comparison and Drilling Parameters ​

Comparison of LNG Plant Components Requiring Deep Hole Drilling ​

Drilling Parameters for Cryogenic-Service Materials ​

FAQ ​

Why is counter-rotational BTA drilling required for long LNG cryogenic pump shafts, and how does it improve bore straightness? ​

Counter-rotational BTA drilling is required for LNG cryogenic pump shafts longer than approximately 6 m because single-rotation drilling (tool rotates, workpiece stationary) cannot maintain the straightness required (typically < 0.1 mm/m) over such extreme lengths (up to 30 m for the longest LNG vertical turbine pumps). In single-rotation drilling, the circumferential component of the cutting force pushes the BTA head against one side of the bore wall. This preferential side loading causes the head to deflect in a consistent direction, creating a curved bore that spirals away from the nominal axis by 0.2–0.5 mm per metre of depth. Over a 30 m shaft, this would produce a bore deviation of 6–15 mm at the far end — orders of magnitude beyond acceptable tolerance. In counter-rotational drilling, the workpiece rotates at 10–30 rpm in one direction while the BTA drill tube rotates at 100–400 rpm in the opposite direction. The relative cutting speed at the bore wall is the sum of the two speeds (110–430 rpm equivalent), producing efficient cutting. Critically, the rotation of the workpiece distributes the circumferential cutting force evenly around the full bore circumference instead of concentrating it on one side. As the workpiece rotates, the "preferred side" of the BTA head changes continuously, and the net lateral force on the head averages to near zero over each workpiece revolution. This reduces the bore deviation from 0.2–0.5 mm/m (single-rotation) to 0.03–0.08 mm/m (counter-rotational). The counter-rotational configuration also cancels the gyroscopic precession of the long drill tube, which would otherwise cause the tube to deflect under its own weight at lengths above 10 m. The practical limitation of counter-rotational drilling is the machine complexity: the workpiece must be mounted in a lathe headstock with a steady rest, and the BTA drill tube must be driven by an independent spindle with synchronised speed control. This requires a machine with two independent drive systems, which is 30–50% more expensive than a single-rotation BTA machine.

What cryogenic proof testing is required for LNG pump shaft bores, and what are the acceptance criteria? ​

Cryogenic proof testing for LNG pump shaft bores verifies that the shaft does not distort when cooled to the operating temperature of LNG (−162°C). The test procedure is specified by API 610 (Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries) and by the cryogenic pump manufacturer's internal specification. The shaft is placed in a test chamber or tank, filled with liquid nitrogen (−196°C, which is colder than LNG at −162°C and therefore provides a conservative test margin), and soaked for a minimum of 4 hours per 25 mm of shaft wall thickness. Before and after the cryogenic soak, the following measurements are taken: straightness (measured by a laser straightness gauge through the centre bore, at 500 mm intervals along the full shaft length); bore diameter (measured by air gauging at 5 positions along the bore); and shaft OD at 5 positions. The acceptance criterion is zero measurable distortion: the change in straightness must be less than 0.02 mm, the change in bore diameter must be less than 0.01 mm, and the change in shaft OD must be less than 0.01 mm. Any measurable distortion indicates that the shaft has retained residual stresses from the BTA drilling process that were not fully relieved by the post-drilling stress relief heat treatment (typically performed at 750–850°C for 2–4 hours after rough BTA drilling and before finish BTA reaming). If the shaft fails the cryogenic proof test, it must be re-stress-relieved and re-tested. The cryogenic proof test is performed on 100% of LNG pump shafts (unlike the sampling-based testing for many other pump components) because a shaft failure at −162°C during LNG pump operation can cause catastrophic leakage of LNG into the environment.

How are cryogenic valve stem bores drilled, and what challenges do the materials present for gun drilling? ​

Cryogenic valve stem bores are gun-drilled in extended valve stems for top-entry and side-entry cryogenic valves used in LNG pipelines. The stem connects the valve actuator (at ambient temperature on top of the cold box) to the valve closure element (immersed in LNG at −162°C at the bottom of the cold box), and the stem must be long enough to pass through the insulation and the cold box wall — typically 1–3 m length. The stem has a central gun-drilled bore (20–60 mm diameter) that houses a valve position indicator rod and provides a leak-tested passage for any fugitive emissions to be safely vented. The stem is made from 316L stainless steel (for shorter stems, where the stem is cooled to near-LNG temperature) or from Inconel 718 (for longer stems, where the higher thermal conductivity of 316L would cause excessive heat leakage through the stem). The gun drilling challenges for these materials are: work hardening of 316L (the material work-hardens at the cutting edge, causing built-up edge that degrades the bore surface finish — mitigated by using a PCD-tipped gun drill with a polished rake face and a feed rate above 0.03 mm/rev); low thermal conductivity of Inconel 718 (11 W/m·K versus 15 W/m·K for 316L and 50 W/m·K for carbon steel — heat accumulates at the cutting edge, requiring coolant pressure above 60 bar to evacuate the heat); and the high length-to-diameter ratio (up to 100:1 for a 3 m stem with a 30 mm bore), which requires the drill to be guided by a steady rest at the stem entry and by carbide guide pads on the drill itself. The gun drilling parameters for 316L stems: Vc = 50–65 m/min, f = 0.03–0.05 mm/rev, PCD-tipped gun drill, high-EP oil coolant at 50–70 bar. For Inconel 718 stems: Vc = 15–25 m/min, f = 0.02–0.04 mm/rev, PCBN-tipped or PCD-tipped gun drill, coolant at 60–100 bar. The valve stem bore is helium leak-tested after drilling (pressurised at 10 bar, leak rate < 1 × 10⁻⁹ Pa·m³/s) to verify that the drilling did not create a leak path through the stem wall.

How is the BOG compressor piston rod gun-drilled, and what tooling is recommended for the hardened 17-4PH material? ​

The BOG (boil-off gas) compressor piston rod is a critical component in LNG storage and regasification systems — the rod transmits the reciprocating motion from the compressor crosshead to the piston inside the compressor cylinder, handling boil-off gas at cryogenic temperatures (−50 to +50°C). The rod is typically 100–300 mm diameter, 1–3 m length, made from 17-4PH stainless steel in the H1025 condition (aged at 1025°F, resulting in 35–40 HRC, UTS 1100–1300 MPa). The rod has a gun-drilled central bore (10–25 mm diameter, full rod length) that supplies lubricating oil to the piston rod packing and the crosshead pin bearing. The gun drilling of the bore in 17-4PH H1025 requires PCD-tipped tooling because the material's combination of moderately high hardness (35–40 HRC) and its tendency to form a built-up edge on carbide tools makes carbide drills uneconomical (carbide tool life is 5–20 m cumulative, versus 30–80 m for PCD). The PCD-tipped gun drill should have a fine diamond grain size (5–10 µm) for the best edge sharpness, a point angle of 130°, an outer clearance of 10°, and an inner clearance of 14°. The drilling parameters: Vc = 25–35 m/min, feed f = 0.02–0.04 mm/rev, high-EP oil coolant at 50–80 bar. The peck depth should be 5–10 mm with a 0.3 s retract dwell to clear the chips. After gun drilling, the bore is inspected by air gauging (the diameter must be within the H7 tolerance, e.g., 20H7 = 20.000–20.021 mm) and by borescope at 50× (no burrs, no surface defects, no chip packing marks). The rod is then ground to the final OD using the bore as the reference surface, with the bore held on an expanding mandrel that centres the rod within 0.01 mm TIR. The completed rod is ultrasonically inspected through the bore (the bore provides access for the ultrasonic probe to scan the full rod cross-section for internal defects) in accordance with API 618 (Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services).

What is the recommended multi-spindle BTA drilling process for LNG vaporiser tube sheets, and what are the quality requirements? ​

LNG vaporiser tube sheets (typically 304L stainless steel or 5083 aluminium alloy, 50–200 mm thick, 1–3 m diameter) require hundreds of precisely positioned parallel bores for heat exchanger tube insertion. The tubesheet is drilled on a multi-spindle BTA drilling machine with 4–20 spindles (depending on the tubesheet diameter and the production volume). The BTA drilling parameters for 304L tube sheets: cutting speed Vc = 45–60 m/min, feed f = 0.10–0.15 mm/rev, carbide BTA head with TiAlN coating, oil coolant at 30–50 bar. The bore diameter tolerance is H9 (e.g., 25H9 = 25.000–25.052 mm), and the bore position tolerance (pitch between adjacent bores) is typically ±0.10 mm for LNG vaporiser tube sheets. The positional accuracy is verified by CMM after drilling (the full bore pattern is measured, and any bore positioned outside the ±0.10 mm tolerance is recorded and evaluated for repair or for tube insertion with a deviation permit). The bore surface finish must be Ra < 2.0 µm for 304L tube sheets (the tubes are expanded into the bores by hydraulic expansion at 200–300 MPa, and the surface roughness creates a mechanical interlock between the tube and the tubesheet that contributes to the joint strength). The tube sheet drilling sequence for a typical LNG vaporiser: (1) spot-face both sides of the tubesheet (removing 1–2 mm from each face to create a flat, perpendicular surface for drilling entry and exit). (2) BTA drill all holes from one side in a single pass (using 10–20 spindles arranged at the hole pattern pitch). (3) Deburr both faces by a light face-milling pass (removing 0.1–0.2 mm, which also removes the drill entry and exit burrs). (4) Inspect 100% of bores by air gauging (diameter) and CMM (position). (5) Clean the bores by high-pressure water flushing at 100 bar to remove all BTA drilling chips and coolant residue. The tubesheet is then ready for tube insertion and expansion.

The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified LNG equipment engineers, cryogenic materials specialists, and pump manufacturers for specific LNG drilling applications. Data and recommendations are based on published research and industry experience as of 2026.