Dry gas seal basics of investing
Today, nearly all new centrifugal compressor installs feature dry gas seals DGS. With fugitive emission rates as high as cfm 5. Both options require a relatively low up-front CAPEX investment and represent the lowest possible emissions approach to reduce and even eliminate fugitive emissions while improving compressor performance. Wet seals use circulating oil as a barrier and were utilized extensively up until the s.
Since then dry gas seals have been installed on virtually every newly manufactured compressor. These seals offer numerous technical advantages over wet seal systems, including: Improved reliability, less maintenance — Compressors with wet seals require ancillary systems and equipment to circulate and treat oil. These systems are often old and need extensive maintenance. If the oil circulating system does fail, the compressor must be taken offline.
This is not an issue with dry gas seals, as complex electro-mechanical systems are not required to support them. The result is fewer failure points and higher uptime; No oil ingress — With dry gas seals, there is no oil ingress risk across the seal into the process gas. Thus, the potential for contamination is eliminated and product quality can be maintained; No parasitic power losses — Oil circulation equipment, such as pumps, require power to run.
Electricity consumption can be as high as kW per hour, depending on the size of the compressor. This is an order of magnitude higher than the electrical load for a DGS system around 5 to 6 kW , and; Reduced emissions — although wet and dry seals exhibit similar leakage rates across the seal face typically between 2 to 6 cfm [0. In compressors with dual wet seals, fugitive emissions rates between 40 to cfm 1. The exact rate is mostly a function of the seal size and operating pressure.
While replacing a new compressor unit equipped with a DGS system would be preferable, it is not a viable option for many across the oil and gas industry whose capital budgets have been slashed amidst the sustained low-price environment. For these operators, a DGS retrofit is a more economical option. To ensure retrofit compatibility for installed compressors, the DGS cartridge is designed to fit any casing cavity using site tooling. DGS panels and boosters can also be upgraded to meet the latest industry standards.
As part of the retrofit, advanced analytics that can assist with diagnostics and digital simulation of the DGS are offered. At the heart of the DGS system is a pair of rings. A stationary ring is held against a rotating ring by spring pressure. The seals are available in multiple configurations, including: Carbon ring process seal - for minimizing clean gas flow and for maintaining a clean process side environment; Carbon ring separation seal - for protecting dry gas seals from bearing oil ingress; Single DGS - for restrictive applications; Double DGS - for lower pressure applications where zero-emissions operation is required, and; Tandem DGS - When a lower emission and high-pressure tolerant solution is needed optional without intermediate labyrinth.
Double and tandem DGS upgrades ensure compliance with all current and proposed emissions regulations, including Europe and the Americas. In certain countries, re-bundling of internals ie re-aero of existing compressors are considered a new installation, and previously established emissions limits are no longer valid.
In this case then end-users need to replace the DGS to meet the latest regulatory requirements. One factor commonly overlooked, he says, is determining when changes in gas properties such as pressure and temperature will cause condensation. For example: A Malaysian petroleum facility operated by a leading American company experienced frequent seal failure.
An investigation revealed that the malfunction was caused by buffer fluid forming condensation across the pressure regulator valve located downstream of the filters. Knowing this, the maintenance team was able to move the pressure regulator upstream of the filter. Subsequently, the team added a heater and insulated the entire line.
The problem disappeared. Based on this experience, as well as numerous requests from the field, John Crane dedicated various resources to developing a gas conditioning unit GCU. Designed with the intent of consistently delivering clean, dry, properly pressurized gas to seals, the GCU has proven to advance the performance of dry gas seals by solving critical buffer gas supply issues.
A heater controller also monitors and maintains gas temperature. Maintaining fluid gas temperature above the dew point prevents condensation of aerosols in the process gas stream. Therefore, the collective features of the GCU effectively manage liquids to ensure that the cleanest and driest possible gas supply is available at all times. The intensifier then provides sufficient seal gas flow to prevent unfiltered process gas from working its way back to the seal faces across the inboard labyrinth, clogging the grooves and causing failures.
With minimal customer interface connections and self-controlled, self-regulated functions, the GCU meets the difficult sealing challenges faced by many in the industry. For example, many maintenance technicians in plants with dry running gas seals have never received essential training in seal operation and maintenance. Instead, training programs should be arranged in conjunction with supplier selection. It is also essential, he says, for the seal manufacturer to provide on-site technicians to help improve equipment reliability, mean time between change or repair, and overall plant productivity.
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By choosing a spiral groove seal with an inside diameter to outside diameter ID to OD gas pumping arrangement, end users benefit from the latest spiral groove developments and lock in dependable, long-lasting equipment operation. Dry Gas Mechanical Seals Unlike typical, liquid-lubricated seal faces, spiral grooved, dry gas seal faces run without face contact. Non-contacting operation eliminates wear, seal face friction and the heat that results from friction.
The most costly component in a liquid-lubricated sealing system is often the seal support system. Liquid-lubricated seals generate significant heat because of face friction and liquid churn from the seal components running in the captive barrier fluid. Along with providing an adequate volume of seal barrier liquid, the removal of seal-generated heat is the goal of the seal support system. By eliminating friction, an effective dry gas seal can greatly reduce the amount of time and resources associated with system operation and maintenance.
Dry gas seals are ideal for sealing volatile fluids in chemical and petrochemical processes, reactive or temperature-sensitive media, oxygen-sensitive processes, and cold applications. Because the seals do not require barrier liquid, they are also well-suited for sealing extremely pure materials with virtually no process contamination.
Spiral Groove Technology Developed more than 40 years ago, the spiral groove dry gas seal remains the standard for effective dry gas sealing. This type seal face features micro-machined spiral grooves. When the pump shaft turns, the spiral grooves compress the pressurized barrier gas and create a lifting force on the opposing face ring. The result is non-contacting operation. With barrier gas at the ID of the face pair, the spiral grooves are designed to pump gas toward the OD.
The stiff ring of compressed barrier gas exiting the face pair creates a highly effective dry seal. Exiting barrier gas passes into the seal chamber at the OD of the inboard seal. Exposed to a large volume of pumpage, the barrier gas entering the seal chamber is unable to dewater the pumpage.
This allows the inboard seal to run in a liquid environment and remain free-moving, eliminating inboard seal hang-up and possible seal failure. Some spiral groove gas seals also feature reverse pressure capability. In the event of a pressure reversal, in which seal chamber pressure exceeds barrier gas pressure, the secondary seal shifts, closing the inboard seal and preventing product release.
Other types of dry gas seals, such as wavy-faced seals, T-slots and deep grooved face patterns, fall short compared with spiral groove technology. While they may impart some gas movement and provide some lubrication and lift, they do not achieve predictable and efficient lift — and the consistent non-contacting operation — that spiral groove dry gas seals offer. Because the space between the seals is less than a human hair, the sealing gas used must be completely dry and free from grit, dust or moisture.
An external source of sealing gas is therefore used to ensure cleanliness. The tandem seal system is designed so that after the failure of the primary seal, the machine can be safely shutdown using the containment provided by the secondary seal without a hazardous release of gas to the atmosphere. Although these dry gas seals are able to handle high levels of vibration without damage, reverse rotation of the shaft at high speeds will damage the seals as they are not able to develop the hydro-dynamic pressure required to push the sealing faces apart.
Seal Gas The supply of filtered buffer gas is injected into the cavity between the primary gas seal and the inboard labyrinth. This supply of gas will leak past the inboard labyrinth back into the compressor which will ensure that the cavity is free from liquids or particles that could damage the gas seal. The supply gas will also leak past the primary gas seal, into the cavity between the primary and secondary gas seals which is directed to an approved vent or flare system.
Labyrinth Seals Labyrinth seals are used in conjunction with dry gas seals in order to restrict the leakage between chambers around the seal A labyrinth seal works like a maze, creating a torturous path which the fluid needs to flow through in order to escape Lubricating oil from the compressor bearings is prevented from entering the dry gas seal by the use of a simple labyrinth seal supplied with separation gas to create a barrier to the migration of lube oil.
Using a system of notches and grooves, the pressure is broken down little, by little so that leakage is minimized not stopped Pressure Balancing The thrust created by the high pressure discharge pushing the compressor rotor back towards the suction is cancelled by creating opposing thrust using a balance drum.
This pressure balancing system also means the compressor seals are only ever exposed to suction pressure! Primary Gas Primary gas is injected in front of the seal to create a positive flow. It is used to ensure the gas entering the seal is completely dry and clean, completely free from dirt, dust, and moisture. Secondary Gas Secondary gas is nitrogen used to provide a clean source of gas to the secondary seal faces.
The 2 seals are separated by another labyrinth and the leakage gas is lead to an atmospheric vent in a safe location above the building Separation Gas Separation gas is nitrogen used to keep lube oil from the adjacent bearing housing from leaking into the dry gas seal system Typical control scheme for dry gas sealing A controller maintains a constant differential pressure above suction pressure, while the flows to each seal are also monitored to check seal integrity.
Seal gas vent The vent system is kept under backpressure bypassing the flow through a restriction orifice. A secondary path is opened by bursting a ruptured disc to safely vent the higher flows caused by seal failure. A differential pressure transmitter monitors the pressure between seal supply and the low-pressure vent.
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