Avoiding Gas Losses

Monitor cryogenic operations carefully to reduce lost product—and lost revenue.

When storing or handling cryogenic gases, there usually will be some venting, leakage or blow down of equipment resulting in unnecessary loss of ever more expensive products. Facilities managers are able to correct or minimize most unnecessary discharges.

gasopschart1

Total product losses can be determined by totaling all invoices in cubic feet of each product of liquid received and subtracting the total shipments of liquid in liquid cylinders and liquid trucks plus high pressure gases. All cryogenic operations will show a loss, especially if several months are considered at a time. This lost product can be reduced and therefore should be monitored regularly. Changes in the lost quantity from month to month can represent changes in performance of equipment or failure to follow established procedures by operators.

Vacuum-Insulated Storage Tanks
Sizing is important and can be determined by estimating monthly usage. Although there is no perfect size or method of determining this because of the many variables associated with each plant’s operation, deliveries of more than once per week or less than once every two months can be a problem. Excess time between deliveries will probably result in loss of product due to exceeding the normal evaporation rate (NER) of the tank. Frequent deliveries may indicate a tank that is too small and possibly result in excessive product pricing. A smaller tank is also less efficient, that is, has a higher NER, as measured in percent of total contents per day.

Tank vacuums are important to watch. High vacuums (over 100 microns when cold) may cause tank sweating and pressure buildup and venting of product. Frost spots can be caused by settling of perlite insulation and/or tears in super insulation, also causing pressure buildup. Vacuum increases to only 200 microns can double the NER of a tank. Tank manufacturers or specialized contractors can help provide repair services.

Tanks that are raised and that have vacuum-insulated jackets down to ground level can provide improved NPSH (Net Positive Suction Head) and insulation of pump piping. This will improve pump life, reduce pump priming time without pressure building, and reduce losses due to excessive pump piping lengths.

If bulk tanks never vent, the tank system is probably operating in a satisfactory manner.

Table 1
Typical Heat Leak and Cooldown Losses for Bare Tubing
  Heat Leak*   Cooldown Loss
    Equiv. Evap. (Cu. Ft./Hr.-Ft.)   Equiv. Evap. (Cu. Ft./Ft.)
Nominal Size BTU/Hr.-Ft Oxygen Nitrogen Argon   BTU/Ft. Oxygen Nitrogen Argon
1 inch 283 37.4 46.8 39.3   18.4 2.5 3.1 2.6
1-1/2 inch 408 53.9 67.4 56.6   33.0 4.4 5.5 4.6
2 inches 533 70.4 88.0 73.9   49.5 6.5 8.1 6.9
3 inches 787 103.9 129.9 109.1   93.0 12.2 15.2 12.7

* Heat leak based on an 8 mph wind blowing over a frosted insulated line.
Cooldown for Type L copper water tubing.

Automated Liquid Cylinder Filling
By automating liquid cylinder filling, operators may perform other duties during filling. Control of back pressure when filling to approximately 30 psi to 40 psi below the bulk tank pressure can reduce filling losses considerably by reducing pressure reduction flash-off and resulting two-phase flow. Many operators open the vent valve wide on the cylinder. This can blow down a cylinder to as low as 50 psi. Throttling the vent valve can result in a settled pressure on the cylinder of 100 psi, after filling from a tank at 140 psi. This difference would result in product savings of 6 percent to 7 percent for argon due to pressure reduction flash-off alone (see Chart 1). Adding the heat leak into piping can easily increase this loss to 15 percent.

A reduction in fill time of several minutes per cylinder also will be experienced because of the reduction in two-phase flow. If 50 cylinders are filled per month, this could result in reducing losses by up to 20,000 cu. ft. to 30,000 cu. ft. per month. This would save hundreds of dollars worth of product per month. Back pressure controls are available from manufacturers, or the operator can throttle the cylinder vent valve manually. If a large number of liquid cylinders are filled at one time, a pump may be considered for filling. This allows filling through the gas phase line, which can result in a no-loss fill and can speed up fill time considerably. A large number of fills are usually required to justify the cost and maintenance of a pump.

Table 2
Effects of Wind, Frost and Ice Buildup on Liquid Lines
The effect of wind, frost and ice buildup on liquid lines is illustrated by the following data for 1-1/8-inch. O.D. tubing.
  Conditions of Transfer Indicated Heat Leak (BTU/Hr.-Ft.)
  No wind and no frost
No wind and 0.1 inches frost
8 mph wind and frosted line
No wind and 0.5 inch ice
No wind and 1.0 inch ice
No wind and 2.0 inches ice
No wind and 4.0 inches ice
160
97
283
264
349
456
550
 

Piping
Liquid lines piped directly to pumps, liquid cylinder filling, etc., will reduce losses or heat buildup in tanks (see Table 1). Pipe fitters like to run lines in squared-off fashion. Angles or more direct routes are acceptable when running cryogenic lines. Low-weight valves, fittings (copper or stainless rather than heavy brass fittings, etc.), tubing instead of pipe, etc., will reduce cooldown and heat leak losses, as can be seen from Table 1.

Table 2 shows that the first frost that builds up on bare piping actually provides a small amount of insulation. Further development of frost and ice on piping that is in continuous service will cause great heat increases in flowing liquid and resulting potential losses because ice is a conductor of heat. If piping is in continuous service, vacuum-insulated lines should be utilized. Manufacturers of vacuum-insulated pipe have computer programs that can estimate savings if flow rates, length, arrangement and in-service times are submitted.

Consider the following when operating equipment:

  1. Control back pressure when filling liquid cylinders as mentioned above.
  2. Fill liquid cylinders of each gas only once per day if possible. This reduces cooldown losses, and the frost that builds up on bare hoses and lines will help insulate and reduce heat leak losses.
  3. Fill cylinders that recently have been in service and are still cold. This reduces cooldown of the inner container of the liquid cylinders, saving cooldown losses.
  4. Leaks in piping, especially of liquefied products, should be stopped. This is especially noticed when the same high-pressure pump is used for argon and nitrogen. The changing of fittings at the pump inlet and gas return are frequent sources of liquid leaks.

Attention to some of these details can result in real dollars saved which, of course, goes directly to the bottom line.

Important Definitions
Saturated Liquid: This is a liquid that will absorb no more heat without boiling unless there is a change in temperature and/or pressure.

Saturation Temperature: For each pressure, there is a precise temperature that marks the boiling point of a particular liquid. The temperature chan-ges with each change in pressure at that given point in the liquid.

Saturation Pressure: The pressure that determines a given saturation temperature point for each given liquid.

Flash-Off: A sudden vaporization of a quantity of liquid. When the pressure above a saturated liquid is suddenly reduced, flash-off (boiling) and subsequent saturation pressure reduction will occur.

Stratification: Stratification occurs when there is a temperature difference between the liquid on the surface in a container and the liquid at the bottom. Liquid cylinders or tank trucks recently filled or transported will indicate a pressure on the tank pressure gauge that represents its saturation pressure. After a few days of sitting, there will be a significant difference in temperature at the top versus the bottom, and therefore its corresponding saturation pressure at the top will be much higher relative to the middle or bottom of the tank. Distributor storage tanks typically will have a saturation pressure at the bottom of the tank 20 psi or more below the tank gas phase pressure.

Two Phase Flow:  When pressure on a liquid is reduced below a liquid’s saturation pressure, rapid boiling will occur. This creates a large increase in volume (when one cubic foot of liquid oxygen boils, it results in the creation of gaseous oxygen of 860 cu. ft. at 70 degrees F and atmospheric pressure) which will greatly slow down flow of liquids as the pressure drops along a length of pipe through valves or into a liquid cylinder vented to the atmosphere.


Gases and Welding Distributors Association

Joseph Higgins Meet the Author
Joseph Higgins is chairman of Abbott Welding Supply Company in Olean, New York, and on the Web at www.abbottwelding.com.