2445 HP Sour Sales Gas Compressor Package

Due to a tight manufacturing window presented by the client, Compass presented a “Cost Plus” arrangement. This allowed major components to be procured 6-8 weeks prior to the order being awarded, and removed a lengthy bidding cycle. Compass was able to move straight into engineering and design, focusing on client specifications and design requirements. 

The completion of this project was 10-12 weeks ahead of the conventional “End User/EPCM/Vendor” procurement cycle, and an open book on estimated labour and material was initially provided to the client, with weekly progress reporting. This provided the client with cost control on a weekly basis.

2445 HP Sour Sales Gas Compressor Package
Compass Compression Model No.	CT2445-RAD4-1 Sour
Application	Sour Sales Gas Compressor
Location	Kobes (Northern BC)
Objective	Install a 2500 HP single stage gas compressor package with shortened engineering and construction schedule time lines.
Design Challenges	Packaging constraints for BC shipping & weight restrictions,  design performance requirements based on limited compression horsepower available from the Caterpillar G3608TALE driver.
Compass Compression Solution	Compass proposed the use of a Caterpillar G3608TALE with an aux-water section rated for 110°F intercooling.  This increased the horsepower rating from the nominal 2370 bhp to 2445 bhp, a 3.1% increase in available HP for little additional capital cost to the project.   The added HP allowed Compass Compression to exceed the design process requirement, while maintaining a costs on the project.
Major Equipment Specifications
Compressor	Ariel JGD/4 four throw, single stage natural gas compressor
Drive	Caterpillar G3608TALE, rated 2445 bhp @ 1000 rpm
Air Cooler	Air-X-Limited model 156 EF forced draft process gas cooler assembly
Control Panel	PLC based control panel c/w accessories

Please contact our technical sales team to determine your compression package requirements.

Compass Compressions Annual Ski Trip 2013

Compass Compression hosted its sixth annual ski trip to Whitefish, Montana the weekend of January 26th.


These trips are a great opportunity to network with industry professionals while taking advantage of the local atmosphere and legendary Whitefish skiing.

Sunshine, fresh snow and almost spring like weather greeted us on Saturday morning and continued through the entire trip and, contrary to the photo below, visibility on the slopes was fantastic.

Whether our group was taking it easy on the groomers, challenging the tree runs or enjoying the on-hill spa everyone had a great time.

Thank you to our sponsors for making it all possible, and to all of our guests for making it a huge success.

2 - 150 HP Rotary Vane VRU Compressors

2 - 150 HP Rotary Vane VRU Compressor Package (Custom Unit)
Compass Compression Model No.	EM150-RV-11S (Dual Train)
Application	Vapour Recovery
Location	Central Alberta
Objective	Multi-source sour vapour recovery application with significant turndown capability
Compass Compression Solution	Compass Compression designed and built a dual train compressor package with the ability to operate a single vane or two vanes in parallel.
Major Equipment Specifications
Compressor	2 Ro-Flo model 11S rotary vane compressors
Drive	2 150 HP, 1200 rpm electric motors
Air Cooler	54VVI forced draft water cooler for process and jacket cooling

Please contact our technical sales team to determine your compression package requirements.

How To Avoid Oil Dilution With High Dew Point Temperature Discharge Gas

By Glenn Schuster

“Oil-flooded screw compressors are great for low suction pressure compressor applications!”  This statement is very much true and screw compressors have found many applications within the Oil and Gas Industry.  However, their advantage over other forms of compression can become a detriment in some low pressure applications.

Why are screw compressors so good for low pressure applications?  During compression, oil is introduced into the gas stream.  The oil is introduced for many purposes: to clean and lubricate the bearings; to clean, lubricate and help seal the rotors.   Beyond these reasons it is also injected to absorb the heat of compression.  This permits many ratios of compression to be achieved in a single stage.  A typical design for screw compressor in natural gas compression will be for 5 to 15 ratios with a design discharge temperature of between 200°F and 220°F (93°C and 105°C).  In these applications, a single stage of compression with a screw compressor can replace 2 or 3 stages of compression with a reciprocating compressor.

When should there be concern over using a screw compressor?  The benefits derived from introducing oil into the gas stream can, in some instances cause operational difficulties.  For example, having the compressor lube oil exposed to the process gas exposes the lube oil to being contaminated by the process stream.  Particulate contamination can easily be filtered out.  However liquid contamination cannot. Liquid contamination can become a concern if the dew point of the gas at compressor discharge pressure is within 40°F (22°C) of compressor discharge temperature. So caution should be exercised if any one of the following conditions are present.

     1) The gas is ‘Hot’.  T_in > 90°F (32°C)
     Hot inlet gas from a source that will be water saturated, ie a well-head, treater, battery,
     tank, etc, will carry a larger percentage of water than it would at a lower temperature,
     thereby increasing the discharge dew point temperature of the gas.

     2) The gas is ‘Heavy’.  SG > 0.90
     As the mole weight of a hydrocarbon gas increases, the discharge hydrocarbon dew
     point temperature increases.  Gases with a high percentage of C3+ should be looked at.

     3) The gas is ‘Sour’ H2S > 10 ppm
     H2S on its own is not a concern for condensation.  However, with the presence of water
     in the gas stream, any concentration H2S creates concerns around condensation and
     the formation of acids.  

Example 1, Effect of temperature on water solubility

Referring to the water solubility graph below from GPSA we can see that at 14.7 psia and 60 °F a saturated gas will carry ~800 lbs water / mmscf of gas.  Keeping the pressure constant and increasing the temperature to 90 °F the same gas will hold ~2000 lbs water / mmscf of gas.



Figure 1

Gas Processors Suppliers Association, GPSA Engineering Data Book, 12th Edition, 2004, Water Content of Hydrocarbon Gas

Example 2, Typical P-T compression curve

In Figure 2, below, we see a representative dew point curve of a heavy mole weight gas. The redline represents a typical pressure/temperature curve of an oil flooded screw compressor. In this example the gas starts out 100% water saturated at about 105 °F and 7 psig at suction. It is then compressed to 150 psig with a compressor discharge temperature of 195 °F. At this point the gas has only 13 °F of super heat which is well below the 40 °F recommended superheat. Running at these conditions water would likely condense on the walls of the oil separator and associated piping systems, contaminating the oil.



Figure 2

Ariel Corporation. Phase Envelope with mark-up.

How can you manage dew point concerns?  To manage discharge dew point concerns proper oil selection becomes critical and control needs to be exerted over the discharge temperature.  So in order of application the means of dew point control are:

 

• Select the proper oil for the application.  A dilution check needs to be performed by the oil vendor.  If they cannot do so their oil should not be used in any situation.
• Control lube oil temperature supplied to the compressor with a 3-way temperature control valve.  As well it is beneficial in colder climates to prevent excessive oil by cooling the oil through an indirect mean (IE.: indirect glycol cooling circuit).
• Increase the discharge temperature by decreasing oil injection flow.
• Maintain the compressor in a warm ambient condition.  This means housing the compressor package in an enclosure c/w heating as required for cooler climates.
• Provide direct heat into the compressor lube oil by means of an immersion heater or external heat source.
• Prevent heat loss by insulating the compressor discharge oil piping, separator and lube oil piping as required.

In addition to the above measures, further care and attention need to be paid at start-up and shutdown.  A compressor that may be operating without issue at steady-state condition can run into trouble during these times.  Measures that can be employed to manage dew point at these times, in no specific order, are:
 

• Blowing down of the oil separator to as low a pressure as possible, either suction pressure or atmospheric pressure at shutdown.
• Introducing a sweet/dry purge gas at either or both shutdown and start-up.

In closing, enough emphasis cannot be placed on managing dew point concerns.  In applications where dew point is an issue and it is not properly addressed a screw compressor can have a catastrophic failure in a matter of hours.  However, in those same conditions when screw compressor package is properly designed and the high discharge dew point issue is appropriately addresses and controlled, a screw compressor package can have many years of trouble-free operation.

So if you have an application that you believe may be of concern with regards to a high dew point gas, ensure that you are talking to an individual with experience in the field and whom can properly address the concern.

Photo - Compass Compression

1478HP Low Pressure Pipeline Screw Compressor Package

 

Why does turning out a cylinder variable volume clearance pocket (VVCP) reduce capacity?

By David Am and Scott Douglas

 

First of all, what is a Variable Volume Clearance Pocket (VVCP)?
 

A VVCP is a device installed on the outer end (Head End) of a double acting reciprocating cylinder that can add clearance to a cylinder by turning the pocket out.

 

Figure 1

Ariel Corporation. Application Manual. Variable Volume Clearance Pocket.

Photo - Compass Compression.

Ariel Reciprocating Compressor.

 

Secondly, why do we use them?

1.       1. For capacity control / horsepower (HP) limitation. This is typically for the first stage (stage-1) cylinder(s) of a single or mutli-stage reciprocating compressor.
2.       2. To reduce rod load or discharge temperature / ratio control on a stage, by shifting ratio to other stage(s).

Before examining how VVCPs affect capacity let’s look at an ideal P-V (Pressure-Volume) diagram. Figure 2 below shows the relationship, in one end of a double acting cylinder, between pressure and volume (piston location).



Figure 2

Ariel Corporation. Ideal P-V Diagram,

assuming no valve losses.

· From point 1 to 2, the suction valve opens because the cylinders internal pressure is less than external suction pressure; this draws gas into the cylinder.

 

· From point 2 to 3, gas compression begins when the suction valve closes.

 

· From point 3 to 4, compressed gas is discharged from the cylinder, when internal pressure is greater than external discharge pressure the discharge valve opens.

 

· From point 4 to 1, the discharge valve closes and suction valve remains closed until it reaches point 1. This is because of the trapped gas in the fixed clearance of the cylinder.

Now let’s look at the effect of opening a VVCP.

By opening the VVCP we add clearance to the end of the cylinder, effectively increasing the re-expansion period because the larger volume of gas trapped in the cylinder needs to move further during the piston stroke to re-expand out to suction pressure.

 

On the compression cycle, the piston also needs to travel further along the stroke to reach discharge pressure as there is more volume to pressurize. The ratio of discharge stroke distance with valve open (or suction stroke distance with valve open) over total stroke distance is the discharge (or suction) volumetric efficiency. The smaller volumetric efficiency (VE) of a cylinder, the smaller the effective displacement. Reducing the VE, reduces horsepower consumption (area inside of curve) and capacity moved.

 

Figure 3

Ariel Corporation. Effect of opening VVCP.

For a gas gathering application, when a stage 1 VVCP is opened, the flowrate of gas (Q) will be reduced and the suction pressure will increase (assuming no pressure restriction / control on wellhead or package). An example of this is shown below with compressor capacity curves with 25, 50 & 75% VVCP shown, overlayed on a production curve. A typical well production curve will produce at a lower rate with more restriction. With effective displacement reduced, the compressor does not pull as hard on the well, resulting in higher pressure/lower flow rate.


Sketch - Care of Compass Compression.

Sketch is not to scale, use only as a representation.