In cross compression and flaring applications for piping system maintenance, there is a high probability and consequence of liquids and debris being encountered. But liquids pose a risk when using traditional compressors. ZEVAC’s Doug Sahm explains the alternative.
Pipeline venting is the release of gas into the atmosphere from a pipeline system, commonly used to complete maintenance and for emergency pressure relief.
Cross compression (vent gas recovery) and flaring are two commonly accepted methods of reducing voluntary blowdown emissions. The normal process for these two methods involves a common overall project flow:
- Isolate the pipe system
- Connect temporary equipment (flare or compressor)
- Flare = drawdown connection only
- Compression = drawdown and discharge connections
- Activate temporary equipment to remove pressurised product from the pipe.
Both methods involve bringing the pressure on the system down to a safe level so that maintenance can proceed. In these pre-maintenance depressurisation applications, there is a near certainty that some degree of liquids will be encountered. These can include water vapour, water, ethane, propane, butane, C5+, glycol/methanol, compressor oil, solvents, biocide, and corrosion inhibitor.
Flaring hazards
In flaring operations, the contents of the pipeline are combusted to remove them. For these purposes, flaring is extended to also include thermal oxidation, both enclosed and open flares, as well as any other combustion driven process.
The hazards of flaring have been studied extensively, and well-documented in the wake of several accidents. In these studies, liquid overflow is routinely cited as the biggest hazard related to flaring.
In the design of flare systems for facilities with controlled environments and known process conditions, a variety of protections exist to prevent liquid carryover in the flare.
In field maintenance environments with uncontrolled sites and unknown process conditions, experienced flare providers will provide very large and/or redundant protections for their flare systems.
However, to great risk, it is commonly observed that in field flaring conditions, protections against liquid carryover are not utilised at all.
It should be noted that even in the presence of liquid knock-out protection, slug flow and thermal effects can get past the typical separator systems and result in flare carryover.
Traditional compressors and liquids don’t mix
The consequences of liquids entering a traditional compressor are high, often resulting in catastrophic compressor failure. The problems with liquids in compressors are well understood, as evidenced by the extensive collection of publications as well as the existence of an entire industry of knock-outs, filter separators, and inlet scrubbers that exist to prevent liquids from passing into compressors, which are found at nearly all stations. Most compressor systems rely on inlet regulation which are not designed to encounter liquids or debris, as well as fuel gas and other control systems that are intolerant of anything other than dry clean gas. Failure of these components (such as an inlet regulator) can result in over-pressurisation of the compressor and other components normally protected by the regulator and controls.
In many cases of compressor failure due to liquids ingestion, the failure of the compressor results in a release of process gas and liquids, posing two additional hazards:
A fire hazard exists as soon as the gas is released after compressor failure, because that gas may be in close proximity to electronics, engine, or other hot parts
The gas release within non-pressure-containing equipment is likely to cause secondary projectiles which may cause additional injuries or property damage.
Most compressor stations are so concerned with the possibility of liquids that they employ multiple lines of defence against liquids, for example: station inlet, slug catcher, separator, suction filter separator, compressor. In addition to this, the compressor unit itself will commonly have another additional liquid knock-out separator immediately in front of the compressor frames as a last line of defence against liquids.
Somewhat paradoxically, in many cross-compression applications (during which process conditions are highly unknown), and during which the presence of liquids is nearly guaranteed, there is commonly no protection against liquid carryover at all.
Reducing the risk with ZEVAC
Unlike traditional compressors, ZEVAC is designed to handle up to 100 per cent liquid flow.
ZEVAC compressors were first developed to solve two-phase blowdowns in the wet gas gathering environment, where various equipment contains up to 100 per cent liquids, but can also be 100 per cent gas or any mixture in between. The equipment is routinely used in NGL, condensate, butane, and propane applications in addition to standard natural gas.
The equipment’s ability to safely handle liquids, gases, and mixed phase fluids is derived from the unique nature of the drive train and compression system.
ZEVAC compressors are linear positive displacement compressors. As opposed to rotary equipment (traditional reciprocating and centrifugal compressors), linear compressors are highly tolerant of liquids due to the compressibility of the air within the drive train.
This contrasts to a traditional compressor’s rotary drive train, which has pistons connected directly to the crankshaft coupled directly to the prime mover, and high speed and inertia, so that a sudden stoppage of the piston due to liquid ingress immediately creates incredibly high hydraulic forces in the compression cylinder.
This in turn stops the piston instantly, which transfers shock loads through the connecting rod, to the crankshaft, and back up the drive train until the weak point is found and failure occurs.
In this same scenario, a ZEVAC compressor cylinder becomes filled with liquid, but the lack of inertia in the drivetrain and the compressibility of the prime mover (compressed air), means the piston stops and the air pressure rises.
This causes the ZEVAC to act as a positive displacement pump until the cylinder is cleared of liquids.
At this point, the ZEVAC is undamaged, the liquids have been processed in and out of the ZEVAC unit, and the gas transfer can resume (or additional liquids can be transferred).
ZEVAC reduces the risk inherent in handling liquids within cross compression and flaring applications.
For more information, visit the website or contact sales@tremcopipeline.com.au.
This feature also appears in the May edition of The Australian Pipeliner.