Hydrocarbon Compression

We invariably cool the compressor discharge prior to dehydration. Unfortunately, natural gas will be reheated—sometimes by 10°F — in a typical gas field dehydration contactor. This occurs because of two factors:
• The circulating glycol may be 70° hotter than the contactor gas inlet temperature.
• The heat of condensation or absorption of the water vapor contained in the wet natural gas must be dissipated into the dried natural gas.

If the glycol contactor is properly designed this temperature rise will not effect dehydration efficiency. However, transmission temperatures will increase.

There are three factors which increase an air cooler’s inlet temperature:
• The compressor valves are faulty.
• The compression ratio has increased.
• High pressure, high temperature natural gas is being produced from the wellhead.

heat transfer surface area) a 10°F increase in compressor discharge temperature may increase the air cooler outlet temperature by 5-8°F.  “Troubleshooting Reciprocating Compressors.” Temperature rises above those obtained from this correlation indicate bad compressor valves (plates or springs broken) or, less commonly, leakage across the piston rings in a double-acting cylinder.

Depending on the compression ratio, a 10°F increase in the compressor inlet pressure will translate into a considerably larger increase in compressor discharge temperature. Thus, it is conceivable that the cooler outlet temperature may increase due to the effect of putting high temperature wells on line.

Whenever finned—tubed cooling bundles are arranged in parallel, as shown in figure 5-2, a potential exists for poor cooling due to gas maldistribution. A low gas outlet temperature from an individual bundle is indicative of lack of gas flow through that bundle. To correct this situation, measure the total pressure drop across the coolers. Next, install restriction orifices in the inlet of each bundle, with openings calculated to double the observed pressure drop. This should bring the outlet temperatures from each bundle reasonably close together. If not, take the tube bundle with the low gas outlet temperature off-line for hydro-blasting of the tube side.

Air flow from a fan will vary considerably with the blade pitch. The pitch is adjustable. To save engine horsepower, an operator may set the blade pitch at 15° during the winter. During the summer, he may attempt to maximize air flow by setting the blade pitch up to maximum—22.5°. Almost all fan cooler blades are adjustable over this range.

Watch for loss of air flow through the finned tube bundle by air by-passing the bundle. Especially in older units, the tube bundle may no longer “square-up” with the fan’s frame very well. Seal the leaking areas with strips of sheet metal.

In southern Texas, the most common cause of reduced air flow is attributed to moths. In their uncounted millions, these tiny kamikazes clog the tube bundle. Along with dust and other assorted bugs, moths must be hydro-blasted from the exterior of tube bundles several times a year.

Most fans are designed for a maximum fan tip speed of 14,000 feet per minute. To calculate the tip speed of the fan, do not calculate the fan rpm, from the pulley size and driver speed. The belts may be slipping. Measure the fan speed directly with a tachometer. Then calculate the fan tip speed as follows:

where
F = Fan blade length, ft.

T.S. = Fan tip speed, ft./min.

If T.S. is less than 14;000 feet per minute, first check the tension of the fan belts. Next, for fans powered via a belt drive from a gas driven engine, determine if the fan speed corresponds correctly to the engine speed:

Fan RPM = Engine RPM X (PDE/PDF)

where PDE = Diameter of the fan pulley
PDF = Diameter of the engine pulley

The smaller the pulley (also called a sheave) the faster the fan speed. A number of standard size pulleys for fans are readily available. For example, if you decided more air flow was needed on a cooler, and the calculated fan tip speed was only 10,000 feet per minute, a smaller pulley could be placed on the fan. For instance, changing a 24″ pulley to a 20″ pulley (both are standard sizes) would increase the fan tip speed to 12,000 feet per minute. The end result of such a reduction in pulley size would then be:

• Air flow would increase by 20% (i.e., linear with fan speed.
• The pressure head developed by the fan should increase by 44% (i.e., fan speed squared).
• The engine horsepower consumed by the fan would increase by 73% (i.e., fan speed cubed).

As the horsepower absorbed by a fan is typically in the three to five percent range of total engine horsepower, the 73% increment to obtain an increase in cooling air flow of 20% is normally not prohibitive. Caution: It is good engineering practice to check with the fan manufacturer prior to reducing the size of the fan pulley.

If the air flow existing from the tube bundle is hotter than the effluent gas, the chances are there is insufficient air flow to properly cool the gas. In particular, if the air temperature blowing out of the effluent end of the tube bundle is only 10°-15° cooler than the effluent gas, lack of air flow is almost certainly the culprit.

Underground gas transmission pipelines are externally wrapped in a protective plastic type coating. Gas temperatures in excess of 130°F to 140″F can cause embrittlement and eventual failure of this coating. For this reason, the usual industry practice is to specify that natural gas discharging into a transmission pipeline be cooled to less than 120°F. Also gas entering a pipeline is cooled to promote efficient glycol dehydration. For example, with an ordinary triethylene glycol dehydration unit, operating at a 900 PSIG contactor temperature, an inlet gas temperature of not more than 125°F is necessary to meet pipeline moisture specifications.

Natural gas effluent from a compressor is typically 150°F to 200°F. Wellhead gas from high pressure wells is also in this temperature range. Most often, gas is cooled in a fin-fan air cooler as shown in figure 5-1. The fan is rotated by a belt drive powered by a compressor’s engine. Alternately, the fan may be powered by circulating high pressure oil.

WHAT CAN GO WRONG

• Air cooling is deceptively simple. For instance, I have encountered
• the following problems while troubleshooting air coolers:
• Air leakage around the tube bundle.
• Fan speed too low.
• Belts loose.
• Fan blade pitch wrong.
• External tube fouling.
• Internal tube fouling.
• Maldistribution of gas in parallel tube passes.
• Excessive number of tubes plugged.
• Pass-partition baffle leaking.
• Excessive gas inlet temperature.