The use of vapour-recovery units in the oilfield is on the rise, for both economic and environmental reasons. Blackmer® Oil-Free Reciprocating Gas Compressors are vital components in many of these installations.
The most obvious positive result of the ongoing oil and natural gas production boom in the US can be seen across the country. At the end of January 2014, the average price at the pump nationwide for a gallon of gasoline was $3.28. By December 2016, the price for a gallon of gas had plummeted. to $2.19. The fall in oil prices has been so profound that the US Energy Department predicted that the lower gasoline prices would allow the average US household to save $750 a year in motor-fuel costs. In addition, households were saving hundreds of dollars more because of the reduction in the cost of natural gas and heating oil for home heating.
While the American consumer is the most obvious beneficiary of the US's historically high crude-oil production, there are many other sectors also reaping the benefits. Beneficiaries include the companies that provide equipment for use in oilfield exploration, production, transport and storage operations.
Specifically, increased production in such prominent shale fields as the Bakken in North Dakota, Eagle Ford in Texas, Niobrara in Colorado, and Marcellus and Utica in New York, Ohio, West Virginia and Pennsylvania, has increased the demand for gathering, transport and terminal systems that can store raw crude oil and natural gas until it can be shipped via truck, train or pipeline for refinement and consumption.
Vapour recovery units
This increase in oilfield activity has also meant a corresponding increase in the amount of vapours that are created and emitted during production, transportation and storage and this is the reason why many operators are keen to install vapour recovery units (VRUs) at their oilfield storage sites. VRUs prevent the escape of these vapours, which are saleable assets but are also potentially dangerous to the environment.
In-the-field storage tanks are used to house raw crude oil and natural gas before they can be transported for refinement. Operators must control the amount of potentially harmful vaporus that are released to the atmosphere from the tanks and are turning to vapour-recovery units as a solution.
This growing use of VRUs means there is also a growing need to understand proper skid-assembly installation, which helps to guarantee their reliable performance. The VRUs only operate at peak efficiency and effectiveness if they are correctly installed. The proper installation of a VRU skid assembly for in-the-field use will optimize the performance of the equipment while eliminating many environmental and maintenance concerns.
A key to effective vapour-recovery unit operation in the oilfield is the skid on which it rests. This skid assembly must be the proper weight and anchored correctly if it is to effectively absorb the shaking forces and moments that are produced by the VRU's compressor.
The challenge
The growth in the amount of vapours that are a by-product of oilfield production activities is set to continue as is the attention that regulatory agencies will be paying to the levels of vapours emitted into the atmosphere and whether or not they can be harmful. That's because many oilfield vapours, compounds like benzene, toluene, ethyl-benzene and xylene, have been classified as Hazardous Air Pollutants (HAPs) or Volatile Organic Compounds (VOCs) by the US Environmental Protection Agency (EPA).
Because of this, the operators of oilfield storage facilities must closely monitor the amount of vapours that are being emitted at their sites (if any) and meet the emission thresholds of the Title V Operating Permit Program of the Clean Air Act, which were put into place in 1990. According to Title V, regulated pollutant thresholds for stationary sources include 100 tons per year (tpy) for criteria pollutants, and from 10 tpy for one HAP or 25 tpy for multiple HAPs.
Oilfield storage-facility operators also must be aware of the EPA's New Source Performance Standard 40 CFR, Part 60, Subpart OOOO, which became law in 2012 and has come to be known as the “Quad O” regulation. Quad O establishes emissions standards and compliance schedules for the control of VOCs and sulfur dioxide (SO2) emissions from storage tanks that temporarily house liquids produced during oil and gas production.
Recovering vapours
This is where the VRU comes in. A VRU is a system composed of a scrubber, a compressor, a driver and controls whose main purpose is to recover vapours that are formed inside completely sealed crude-oil or condensate storage tanks. During the VRU's operation, the controls detect pressure variations inside the tank and then turn the compressor on and off as the interior pressure exceeds or falls below pre-determined settings. When the compressor is running, it passes the vapours through the scrubber, where any liquid is trapped and returned to the tank, while the vapour is recovered and compressed into natural gas lines.
For use in the oilfield, the components of the VRU are generally installed on a skid assembly, and the skid can be easily moved and installed as one complete unit. Operational problems arise when the skid is not installed or anchored properly to the ground. Most issues during the operation of a poorly-installed VRU skid show up at or near the compressor, which is the heart of the VRU system.
It is a given that all reciprocating-type compressors will produce some shaking forces and moments. These can be caused by a wide array of operational characteristics, including speed, height, cylinder orientation, intake/discharge pressures, single- or double-acting operation, compression ratio and application conditions like gas composition, or site, placement and environmental variations. If those forces or moments are not properly absorbed into the mounting or foundation of the operating system then vibration of the compressor can occur.
All reciprocating-type compressors will produce shaking forces and moments. If the compressor is not installed correctly, those shaking forces and moments will create vibrations that can harm the performance of the compressor and VRU.
Equipment for oilfield activities
US manufacturer of equipment for oilfield activities, Michigan-based Blackmer, has been building a reputation as a manufacturer and provider of equipment that can reliably be used in oilfield activities. Its reputation extends to its various lines of Oil-Free Reciprocating Gas Compressors, namely the HD, HDS and NG Series models. All can effectively be used as part of an in-the-oilfield VRU that can optimize operational performance and production while reducing costly downtime and maintenance.
However, as noted above, the VRU base and skid assembly must be installed correctly. Blackmer has recently noticed that a number of reciprocating compressor oilfield VRU installations are not operating at peak performance. After investigating these issues, it was found that the problems were not being caused by any individual VRU components; instead the skid assemblies on these installations had not been properly designed and/or installed. Prolonged operation with an incorrectly designed skid or poor foundation can damage the compressor and compromise the VRU's overall operational effectiveness and reliability.
To help educate the industry and optimize the performance of storage-facility VRUs and their components, Blackmer has produced a video entitled, Does Your VRU Compressor Vibrate? It Shouldn't. The video has been designed to explain and illustrate the correct way to prevent vibration in the skid assembly. Proper skid assembly is paramount because when operating at peak speeds, reciprocating compressors can produce unbalanced forces.
Specifically, the short video, which was produced with the help of Electronic Design for Industry (EDI), at its test facility in Ohio, spells out the following steps that should be taken and parameters that should be met for proper skid assembly:
- The compressor should be anchored to a baseplate (or skid) that is at least four times the compressor's weight
- The baseplate with the compressor and other VRU components should be bolted to a concrete slab/pad
- The concrete slab/pad should be situated on a level surface
- The pad should be prepared and graded, if necessary.
- The baseplate skid should never be installed on non-compacted soil.
If these simple rules for skid-assembly installation are followed, the amount and severity of the forces and moments that occur during compressor operation will be minimized, resulting in almost vibration-free operation that will help optimize the VRU's performance and longevity.
Conclusion
In the fast-growing oil and gas industry, speed, portability and reliability are key factors in optimizing production times and the bottom line. There is now an almost constant need for the installation of VRUs in the oilfield as production operations continue to accelerate. VRUs themselves are complex, highly engineered systems designed for reliable performance in a variety of harsh operating conditions. However, the oilfield's rugged terrain, combined with the need for rapid deployment and a reduction in site-time preparation, can compromise VRU installation. OEMs and system fabricators who follow the tips in Blackmer's video and take the time to install the VRU's skid assembly correctly will find that any subsequent time and cost incurred because of downtime, repairs and maintenance will be greatly reduced.
