Hydraulic Power Recovery Turbines

1. Purpose

This procedure defines Inflection Point Engineering's status regarding the inclusion of Hydraulic Power Recovery Turbines (i.e., a pump acting in reverse to recover power by throttling liquid through hydraulic power recovery turbines) in new and revamped unit designs.

2. General

This procedure covers hydraulic power recovery train design, selection, and control. The basic objectives for a successful installation are process stability, reliability, efficiency, flexibility, ease of maintenance, and payout.

Consider hydraulic power recovery turbines when applicable in high pressure hydroprocessing units to drive the charge pump and/or lean amine pump.

3. Secondary Driver Selection

Typically, a motor is used as the secondary driver rather than a steam turbine due to the simpler control scheme and the motor's ability to provide excellent speed regulation. A steam turbine however can be used but is not addressed in this procedure. All new designs shall have a full-size motor capable of providing the rated pump BHP. This allows for continuous unit operation at rated conditions if the hydraulic turbine, which is connected to the train by means of a one-way clutch/coupling, is out for repairs.

4. Characteristic Performance

The characteristic differential head and BHP curves as a function of flow are shown in Attachment 1, Figures 1 and 2. These curves illustrate the rapid decay in differential pressure and efficiency with decreasing flow, which is typical for this type of device. Figure 1 indicates that the hydraulic turbine is designed for a specific set of hydraulics. Processes normally require flow variation. The control scheme ensures a constant differential pressure across the hydraulic turbine with a control valve in series with the turbine will throttle the pressure not absorbed by the hydraulic turbine at reduced flows. With a constant pressure profile across the turbine, it will not accept more than design capacity. The turbine must be modified internally, thus changing its orifice factor to accept higher capacities.

Figure 2 indicates that the BHP recovered has an efficiency of zero at approximately 40% of rated flow. Energy must be added to the hydraulic turbine in order for it to rotate at flow rates below this zero efficiency capacity. Since the turbine is connected to the motor and pump by means of a one-way clutch, the turbine will not be a drag on the train at reduced flows and will engage with the train at approximately 40% capacity and begin contributing horsepower.

5. Mechanical Design Considerations

5.1 Due to high axial thrust during normal operation and the possibility for high transient axial pulses, Kingsbury type thrust bearing, hydrodynamic radial bearings, and a forced feed lubrication system are standard.

5.2 In general, vaporization may occur in the inlet and will normally occur extensively at the exhaust, necessitating pressurized dual mechanical seals with a diesel oil barrier fluid.

• For some multi-stage turbines, especially above 1000 psig pressure level, the casing inlet may be designed for design pressure and the remaining portion designed for a reduced pressure. In addition, mechanical seals, which normally seal against a pressure slightly greater than exhaust pressure, normally are not suitable for the higher inlet pressure. For these reasons, the block valve on the exhaust of the turbine shall always be opened before the inlet valve. In some cases, a relief valve may be considered to protect against overpressuring the casing when the turbine casing has been segmentally designed for different pressures.

• As with centrifugal pumps, high temperatures, erosive and/or corrosive fluids, high pressure, etc., require special consideration when determining the appropriate materials and design of the hydraulic turbine package. If hot separator liquid is used to drive the power recovery turbine (typically for Hydroprocessing Charge Pump applications), check the turbine casing material for susceptibility to high temperature hydrogen attack. The hot separator liquid contains hydrogen which deteriorates the mechanical properties of carbon steel at high temperatures. Once the hydrogen partial pressure of the liquid is known, the attached “Nelson” curve, Figure 4, may be applied to determine if a carbon steel casing is immune to decarburization. If not, specify an 11-13% Chrome casing.

6. BHP Recovery Calculation

Hydraulic turbine efficiency may vary from approximately 50% to 85%. Use the following formula to determine the horsepower recovered by a hydraulic power recovery turbine:

Recovered Horsepower = SPEC.GR. X DIFF. HEAD (ft) X GPM X EFF.

3960

= P (psi) X GPM X EFF

1714

Note that recovered power is proportional to efficiency, not inversely proportional as in pump power calculations.

7. Control

The control system for the hydraulic power recovery train must provide the following:

Stable process control, ensure safe and reliable component operation.

Offer the required operational flexibility to satisfy the full pressure-flow range for all cases.

Be capable of operation minus the hydraulic power recovery turbine.

Recover the available power over the normal expected operation range.

The following control system philosophy satisfies these criteria:

• The control valve on the inlet of the turbine (reference Attachment 2, Figure 3, control valve A) must be capable of handling flows from approximately 50% to 85% of the total flow from the separator. Use a 100 PSI pressure drop at 85% of design flow wide open across valve A for a 2000 psig class suction and a 50 psi pressure drop for a 750-1000 psig class suction to ensure that this valve is properly selected to be capable of stable operation for the full flow range. If the pressure drop for valve A is too low, the selection will be a valve, which is too large and potentially will give poor control at reduced flows.

• The split-ranged level control assembly (reference Attachment 2, Figure 3, control valve B) serves as a bypass and provides a means for handling additional flow required to retain a constant level in the separator. Valve B is sized for passing the remaining 15% of normal total flow from the separator and is capable of handling the total separator flow in the event that the hydraulic turbine is out of service. Size valve B tightly at design flow to handle 15% flow to ensure that the valve is sufficiently open to avoid chatter and wire drawing problems.

• For installations where the total pressure drop from the separator to the turbine exhaust is approximately 1000 psi or greater, replace the globe valve bypass for valve B with a handwheel-operated control valve body (valve C). Size valve C so that the design flow from the separator requires roughly half of the valve capacity or opening. Valve C may be provided with a diaphragm actuator and piped up so that the level controller signal to valve C controls separator level when valves A and B are removed from service.

• On increasing level in the separator, valve A and the hydraulic power recovery turbine continue to pass 85% of the normal separator flow with valve B opening to handle the increased capacity. When the level in the separator falls below the level control setting, first valve B will close, and then valve A will throttle to maintain level in the separator. Reference Attachment 1, Figure 1, which indicates the reduced turbine differential pressure which must be compensated for by the closing valve in series, valve A, to estimate the amount of additional pressure which must be throttled at reduced flow rates.

• An overspeed protection device is included on the hydraulic turbine to prevent damage from a "runaway" condition which may be caused by coupling/shaft breakage, increased differential pressure due to a process upset, a decrease in power absorbed by the connected pump, etc. Inflection Point Engineering installations in the past have used Foxboro Type 16A and Coppus Sentry overspeed trips. In addition, any automatic shutdown for the hydraulic turbine-driven pump must also shut down the hydraulic turbine.

This scheme provides the necessary process stability, flexibility concerning operation and maintenance, reliability of individual components, and safety. The recovery of available energy is maximized within these constraints.

8. Economic Justification

Establish a total installed cost for the power recovery turbine to compare to the utility savings for an economic justification. Obtain the capital cost of the power recovery turbine and clutch from cost estimating or a rotating equipment specialist. Typically, a vendor is consulted for an accurate price. Besides the initial cost of the equipment, there are many other associated costs to be considered including installation, foundations, piping, controls and instrumentation, valves, welding, design and engineering. A good factor to use to establish the total installed cost is 2.5 times the cost of the turbine. For example, if the turbine costs 10, the installed cost would be 25. Compare this number to the cost savings of the recovered horsepower calculated in Section 6 to establish a payout.

The refinery's cost of electricity must be known for an accurate evaluation. Typically, this cost is found in the Basic Engineering Design Questionnaire (BEDQ). The higher the refinery's electrical costs, the quicker the payout. Consider anticipated rising (or dropping) electrical costs. Consider that normally the evaluation assumes running the power recovery turbine continuously at its design point. At reduced plant operation the recovery efficiency quickly decays and the justification for the inclusion of this device becomes extremely difficult.

Typical Hydraulic Power Recovery Turbine Characteristic Head and Recovered BHP Versus Capacity Curves

Figure 1

Figure 2

Typical Hydraulic Power Recovery Turbine Control Schematic

(Detailed Piping, Instruments, Valves, etc. excluded for simplicity)

Figure 3

Valve "A"

Valve "B"

Valve "C"

Required when Valve "B" pressure drop is ~ 1000 psi

Operating Limits for Steel in Hydrogen Service

To Avoid Decarburization and Fissuring

Figure 4