Reprinted from May 2016 LNG Industry
Tyler Brower of Ebara International Corporation, Cryodynamics Division USA looks at current advancements in optimizing cryogenic liquid expander technology.
Cryogenic liquid expanders were first introduced to the market in the late 1990’s as a means to improve the efficiency of the liquefaction process for cryogenic fluids. Since that first expander installation, Ebara International Corporation (EIC Cryo) has worked in partnership with its customers to extend liquid expander technology to new processes and applications, as well as improve the expander equipment itself.
The first EIC cryogenic liquid expander was installed at Oman LNG as a means to replace the Joule-Thomson (JT) valve in the letdown process. This replacement of the JT valve has since proven to increase LNG production by 3-5%. Subsequently, the additional LNG translates to a financial return that makes the liquid expander extremely attractive. Designed in a downward flow configuration, the original expanders were delivered equipped with either variable or fixed speed drives. Over the years, the equipment has seen great success and cryogenic liquid expanders have become semi-standard equipment in new LNG liquefaction plants. Although, over the years, it has become evident that the applied speed-control technology requires not only further optimization, but also the ability to conform to real-life processes and operator limitations for LNG facilities. This article will explain these current advancements and how they are applied.
Innovation: Upward Flow & Two-Phase
A substantial innovation in expander technology came in 2003 with the first installation of two-phase liquid expanders. These units were installed into a nitrogen rejection facility with expander outlet pressures reaching atmospheric. While a single phase expander will drop pressure through the hydraulic set to a pressure above the vapor point of the liquid, a two-phase expander can drop pressure to the vapor point of the process fluid. This allows for full expansion of the liquefied gas through the cryogenic expander, creating additional liquid production improvement in the letdown process over the standard single phase cryogenic expander. This progression to a two-phase design provides an additional 1-2% improvement in in production over the single-phase.
Another notable development incorporated into the two-phase expanders was the upward flow configuration. While downward flow has been proven in the field with many hours of reliable service, upward flow offers several additional benefits to further optimize the liquefaction process. The upward flow design allows for lower outlet pressure due to its greater tolerance to vapor at the outlet, creating more power generation and liquid recovery from the process flow. It also provides more efficient thrust balancing and greater resistance to debris in the fluid. These modifications have proven to provide a robust, efficient machine that also improves the mean time between overhauls (MTBO) of the units. The upward flow arrangement has been proven in testing which led to its implementation in several new installations including Prelude FLNG.
Case Study: Optimizing the Process
Given the constant price changes in the modern LNG market, it is increasingly important for LNG equipment to be as flexible and efficient as possible. Since the first cryogenic liquid expander, the units have been provided with the option of either a fixed speed unit (50Hz or 60Hz) or a variable speed drive system. The variable speed operation is accomplished through the use of either a low or medium voltage variable speed constant frequency (VSCF) drive unit, where as a fixed speed unit’s R.P.M. is set by the frequency of the plant power grid (3000 or 3600 R.P.M.). A VSCF drive allows the expander unit to be flexible and operate under changing process conditions and fluid densities, while still ensuring the generator outputs the correct frequency to the plant operating grid. In a liquefaction plant with the possibility of changing operating conditions the VSCF option provided our customer with the ability to change their liquid expander operation to provide the best liquid recovery in their let down process.
However, when reviewing the operation of the currently installed variable speed liquid cryogenic expanders with some of plant operators it was revealed that the VSCF drive units were not always being used properly. When a process condition would change in the plant, the operators were not always changing the speed of the liquid expander. This means that while the liquid expander may be operating at a safe condition, it would not be operating at the optimum speed for maximum liquid production. The liquid expander speed was not being modified to meet the change in process conditions because either (a) the operators did not fully understand the changes needed, or (b) were fearful of making mistakes and affecting plant operation. With the main purpose of the liquid expander being to increase liquid production in the liquefaction plant, this small change in the process can have large impacts to the plant output in terms of liquid produced and therefore revenue amounting upwards of millions of dollars per year.
As a result of these discussions and internal case studies, EIC has begun to pursue the next phase in cryogenic liquid expander optimization: self-tuning of the expander R.P.M. through the VSCF drive unit. With this optimization option, the adjustment of the liquid expander R.P.M. would be automated and removes the responsibility of the plant operator. The result is the expander will always run at the optimum condition to produce the highest amount of liquid for each process duty point, without requiring the attention of the plant operator.
Expander Optimization Through VSCF
Reviewing some operational curves of both fixed and variable speed expanders can demonstrate how things may change during the liquefaction process and how speed optimization can help improve the production of a liquefaction facility. [Graph A] represents a standard liquid expander operating at a fixed R.P.M. with a set duty point at “Rated Point”. In this configuration, the flow rate (Q) and head pressure drop (m) can be altered with process valve equipment as long as the new duty point still falls onto the fixed speed curve. No R.P.M. changes can be made with this configuration. A fixed speed liquid expander would be applicable to a liquefaction facility where both the process and liquid are expected to see very little fluctuation during plant operation.
The operational curve shown in [Graph B] represents a variable speed expander with a rated point and two additional points inside of the “operating envelope”. Each curve on the Flow vs. Head graph represents a different operating R.P.M. to accommodate the associated duty point. In this operation when the process conditions change the operator must alter the R.P.M. of the expander to ensure the machine is reaching the full potential of additional liquid production. This requires that the operator must know the required R.P.M. for a predetermined duty point. If the new process duty point does not fall onto a point with a known speed, the operator can adjust the R.P.M., flow control valve, and backpressure valve to find what appears to be the best operating point for that condition.
During these process changes at liquefaction plants, it has been found that the operators are comfortable with modifying the flow and pressure through the expander using valve position changes, as opposed to adjustments to the VSCF drive speed. In this situation the expander essentially becomes a fixed speed unit. The VSCF unit will adjust the load on the generator to maintain the set rated speed (normally 3600 or 3000 R.P.M.), however the flow and head will be restricted to those that fall on the rated speed curve. This means that either flow must be diverted or pressure adjusted through the expander in order to meet the new process condition. This is represented in [Graph C].
In [Graph C] we can see the Rated Duty and two additional duty points, Duty 2 and Duty 3. Duty 2 has a higher required flow and head pressure drop than the rated duty point, and duty 3 has a lower head pressure drop than the rated duty point. With proper R.P.M. adjustment from a VSCF drive with operator input, Duty 2 would operate with an increased R.P.M. over the rated point, and Duty 3 would operate with a lower R.P.M. However, without proper speed adjustment from the operator, the only resort is to manage the pressure and flow through process valve adjustment or the hydraulic limits of the expander itself. In order for Duty 2 to operate on the rated duty curve, the pressure through the expander must be reduced. For Duty 3 to operate on the Rated Duty curve, flow will need to be reduced, either by bypassing flow to a JT valve loop or the natural restriction of the expander hydraulics. The downside of this method is that there is now less throughput of expanding liquid through the expander, either by reduction of flow or pressure. Any fluid that does not go through the expander must complete the let-down process through the less efficient J-T valve. The portion of the expanding liquid completing the let-down process through the J-T valve will experience a higher rate of vaporization, resulting in lower liquid production and loss of revenue. This is demonstrated below in [Table 1].
The rated case in [Table 1] shows a typical duty point in a LNG liquefaction let-down process. For this case study the rated case is operating at 3000 R.P.M. (50Hz). The lowered R.P.M. case would be typical of a process change where the head pressure drop through the expander is reduced from the rated case. With proper adjustment of the VSCF drive the R.P.M. would be reduced to 2168 R.P.M. to accommodate this new process condition. If the R.P.M. is not adjusted, then the capacity through the expander must be reduced from 1500m3/hr to approximately 1378m3/hr in order to operate at 3000 R.P.M. as rated. If we examine the reclaimed capacity of the lowered R.P.M. vs. reduced Capacity duty points we can see that not properly adjusting the liquid expander R.P.M. means a loss of approximately $919,000usd per year (at $6.5usd per mmBTU and $.12 per kW-hr). Reviewing the increased R.P.M. case in the same manner we see an even larger drop in revenue from lack of R.P.M. adjustment. This case would be an example of a situation where the head drop across the expander would remain the same as rated, but the capacity through the expander would be reduced. In this situation the speed would be adjusted to 3780 R.P.M. to meet the new capacity. If the VSCF speed adjustment is ignored, then the pressure drop through the expander must be reduced to again meet the 3000 R.P.M. operation curve. At the same LNG and electricity cost a listed above we can see that not adjusting the R.P.M. to meet the new process condition means a loss of $1,967,000 of revenue per year.
An interesting item to review from [Table 1] is that even though in the adjusted head and flow cases the efficiency may be better, the reclaim capacity is reduced. This demonstrates the value of running as much capacity and pressure through the expander by adjusting R.P.M. as opposed to bypassing through valve adjustments. In examining these cases we can see that as the generator power output increases, the amount of reclaimed liquid also increases. This is logical if we review the equation for reclaim capacity based on work (kW) and Enthalpy of Vaporization (kJ/kg). As the fluid remains constant, the Enthalpy of Vaporization also remains constant, so any increase in kW output is directly related to the amount of additional fluid output during the liquid expander let-down process. See [Equation 1].
Applying the Solution
Using this relationship between work done by the liquid expander on the system to the amount of additional fluid produced in the let-down process leads us to the next step in liquid expander optimization. EIC Cryo is working with an industry leading VSCF drive vendor to equip existing drive units with the capability to automatically adjust expander R.P.M. without operator maintenance. Using technology that is already in place for reducing power consumption through VFD driven motors, modifications to the speed of the expander can be made to accommodate process conditions as they happen. This will ensure that the end user is getting the best possible reclaim out of their liquid expander equipment.
This optimization will be accomplished with the main input to the optimization software being the kW output of the generator. As described above in [Equation 1], the kW output is directly tied to the reclaim capacity of the liquid expander. The software will be given an operating envelope as shown in Graph B, with a high and low R.P.M. limit, a low flow limit to the left of the envelope, and a generator power limit to the right of the envelope as a “safe zone” of operation. The optimization software will continuously monitor the generator power output and if process conditions cause a change in the kW output, the software will modify the operating R.P.M. This gives the drive unit the ability find the best possible power output, and therefore reclaim capacity, for the new process condition.
For the liquefaction plant end user and contractor this new optimization program will be a seamless integration into new liquefaction plants, and easily retrofitted into existing liquid expander installations. The software is installed onto already existing hardware within both medium voltage and low voltage drives, and no additional hardware is required for the expander itself. The only requirement for control input the measurement of generator power output, which is already in place at most liquid expander installations.
Much like the move from standard carburetion to electronic fuel injection on modern automobiles, EIC’s optimization software is the next development step in liquid expanders and will allow the precise control of the equipment in the liquefaction process under varying conditions. This ensures the equipment is always operating with the best possible performance, and providing the highest possible level of liquid production for the liquefaction facility.