Turnaround technologies for natural gas compressors

Business is booming for oilfield and refinery overhauls of natural gas compressors

Given its location in Corpus Christi, Texas, H&S Valve is ideally situated to service both oilfield drilling and extraction customers as well as refineries. For natural gas, which must be compressed for distribution through pipelines downstream to up, that means repairing and reconditioning the reciprocating gas compressors that are required for this task.

It also puts the firm in a situation to absorb the shifts and changes in natural gas prices. With natural gas prices low (as they are now at US$1.70 per ft3) that means an increase planned shutdowns and overhauls of equipment on the refining side; when natural gas prices increase – something predicted for 2021 due to dropping oil and gas prices – more requests will come in for overhauls of equipment used at the drilling site. That essentially means “business as usual” for companies such as H&S Valve, which is still overhauling approximately one compressor or more each day.

“The refineries are telling me that now is a prime time to do a turnaround because of the price of natural gas is low, and they can afford to do a turnaround because they aren’t losing money to do it,” says Joe Salinas of H&S Valve.

H&S Valve repairs both slow speed (300-400rpm) and high-speed (1,200-1,400 rpm) natural gas compressors used to pump intermediate and end-product gases in pipeline service or within the refinery. Although there are varying types of compressors, common reciprocating compressors use an electric motor to turn a crankshaft that causes a piston to rise and fall within the cylinder to compress the air or gas. To maximise compression, there must be a tight seal between the piston rings and cylinder bores.

Repair and reconditioning options for natural gas compressors

To accomplish this, H&S Valve repairs valves, rods, liners and starters. However, most of the work involves reconditioning worn, undersized or scratched cylinders from 2-32in and replacing the piston rings. When this is the case, the choice is to oversize the cylinder and install a larger piston and rings or install a cast-iron liner if the compressor. Salinas says many customers opt for liners, given the cost savings. “Customers don’t want to spend US$40,000 for a cylinder when they can get it lined for US$18,000 or US$20,000 and it will work like it is brand new.”

Although many OEMs recommend an overhaul at 10,000 hours, Salinas adds that some customers push the compressor to 15,000 hours or more. “When they bring the compressor in, everything in the cylinder is completely worn out. With new liners, they can start the runtime all over again.”

The first step in the process is machining the bore, followed by rigid honing to get it to the proper geometry and size. A Flex-Hone tool is used for final finishing and to create a cross-hatched surface, which is integral to ensuring that proper oil retention is always maintained in the cylinder bore walls.

As in any motor or engine application, it is particularly important that the piston rings maintain 100% contact with the walls of the cylinder bores. Otherwise, performance problems such as compression blow-by and oil leakage can occur, reducing efficiency. Providing the needed consistent contact between piston rings and cylinder almost always requires honing, either because the bores are not perfectly round, have been damaged by previous ring or piston failure, or due to other wear factors.

After rigid honing of the cylinder bore, there can also be torn, jagged metal particles attached to the cylinder wall surface that are microscopic in size. If not removed, those metal shards and particles could result in piston scuffing problems and possibly some vertical scratching in the piston wall, which could add to oil consumption.

How does the honing tool work in finishing natural gas compressor machinery?

The Flex-Hone removes loose, cut, torn and folded material on the surface of the cylinder, maintains valley depth for oil retention and removes peaks that would otherwise damage rings or seals. The process produces a surface with greater bearing area which increases both the life of both the cylinders and the piston rings. The result is that more lubricant is retained with less seal wear, improving engine efficiency and performance.

Salinas says H&S Valve has been using the Flex-Hone from Brush Research Manufacturing for more than 20 years and has a full line of tools from 2 to 20in to accommodate various cylinder sizes.

Characterised by the small, abrasive globules that are permanently mounted to flexible filaments, the Flex-Hone is a highly specialised abrasive tool available in various grit sizes and diameters up to 50+in (4mm to 36in are standard sizes). Whether for smoothing out rough areas and high points, deburring, edge blending, cross-hatching or removing cut, torn or folded metal, a flexible cylinder hone can be an indispensable tool.

“It is a really handy finishing tool,” says Salinas. “We use the Flex-Hone 90% of the time, because our customers want a crosshatch in the cylinder because they know the piston rings are going to seal better that way.”

The tool is also ideal for cylinders that required deglazing. Not only does the flexible hone set up quickly, but a few passes up and down inside a cylinder liner also accomplishes thorough deglazing.

“If a cylinder needs deglazing, we apply a contact cleaner and use the Flex-Hone to remove all the hardened material and it makes it look brand new again,” says Salinas. “If that build-up is there it will affect the seal, you need virgin material.”