GRC Transactions, Vol. 40, 2016

815

Off Design Operation of Hybrid Noncondensable Gas Removal Systems for Flash Steam Cycle Geothermal Power Plants

Jason Devinney

Project Engineer, Vooner FloGard® Corporation, Charlotte NC, USAjdevinney@vooner.com

KeywordsNoncondensable gas, NCG Removal System, steam jet air ejector, liquid ring vacuum pump, hybrid vacuum system, off design

ABSTRACT

The Noncondensable Gas (NCG) Removal System for the condenser of a flash steam geothermal power plant must be able to remove the NCG load at all plant operating conditions. Hybrid vacuum systems, incorporating steam first stage jet air ejectors (SJAE) and second stage liquid ring vacuum pumps (LRVP) have proven to be reliable and efficient in this service.

The design basis for the NCG Removal System is determined by the conditions corresponding to the highest ex-pected vapor mass and volumetric load. As SJAEs are substantially constant mass flow devices and LRVPs are positive displacement near constant volumetric flow devices, any variations from the design basis will have different effects on the NCG handling capacity and loading of each stage. Since the SJAE and LRVP operate in series, limitations or excess in capacity of each stage will affect the other. The system must be designed so that the NCG Removal System will oper-ate at expected off design conditions.

This paper provides an overview of the operational limits of a typical NCG Removal System at several combina-tions of off design basis conditions.

Introduction

Flash cycle geo-thermal steam includes a component of naturally occurring noncondensable gases typically in the range of 1-12% by weight in the flash steam. Additionally in barometric condenser applications, a significant amount of air is released from the condenser cooling water. The NCG Removal system is designed to com-press these gases from the condenser operating Figure 1. Typical NCG removal system.

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pressure to atmospheric pressure for release. Typically a hybrid system consists of a first stage steam jet air ejector (SJAE) followed by an intercondenser used to minimize the condensable vapor load and finally a liquid ring vacuum pump (LRVP) as shown in Figure 1.

The system requires steam to drive the SJAE, cooling water for the intercondenser and LRVP, and electrical power for the LRVP and control system. The actual configuration of hybrid NCG system often consists of multiple parallel partial capacity trains which may be used with a common or separate intercondenser(s). Configuration is based partially on cost, redundancy requirements and user based confidence in expected NCG load. Systems design basis often exceeds 100% in varying configurations (example 3x50%, 70% + 40%, etc.)

Steam Jet Air Ejector Overview

A SJAE uses a convergent/divergent nozzle to create a venturi effect. The converging nozzle converts the pressure energy of motive steam to velocity energy. This change creates a low pressure zone inside the ejector that draws in the suction fluid (in this case NCG from the condenser). The diverging nozzle converts the velocity energy back into pressure energy. Figure 2 shows a typical ejector.

SJAEs are typically selected to operate at a single design basis compression ratio. At off design compression ratios the suction vapor mass flow rate of the steam jet will vary. Figure 3 shows a typical SJAE mass flow capacity and com-pression ratio curve.

Liquid Ring Vacuum Pump Overview

A LRVP uses a rotating impeller with multiple chambers to mechanically compress the suction fluid from the inlet of the pump to the discharge of the pump. Figure 4 shows a cross section of a typical LRVP.

The LRVP is a positive displacement device that operates at a near constant (other than efficiency loss at higher compression ratios) volumetric flow. The LRVP volumetric capacity curve is typically published as volumetric flow of dry air at standard conditions, the actual operating conditions must be adjusted back to dry air at standard conditions for selection. LRVP power consumption curves are also published for dry air and must be adjusted for compression ratio and vapor density. Figure 5 shows a typical LRVP suction volumetric flow capacity curve.

Intercondenser Overview

The intercondenser uses cooling water to reduce the temperature and water vapor flow between the SJAE and the LRVP. The more efficient the intercondenser, the less volumetric capacity is required in the LRVP.

Design

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Suction capacity, Kg/hr Equivalent Dry AirFigure 3. SJAE performance curve.

Figure 2. Steam Jet Air Ejector (SJAE).

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Figure 4. Liquid Ring Vacuum Pump (LRVP).

Figure 5. LRVP Capacity Curve.

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In a majority of systems the LRVP inlet manifold is equipped with spray nozzles diverting a portion of the required sealing water to provide a further amount of water vapor condensing before the LRVP inlet.

Hybrid System Operation

Hybrid NCG removal systems are typically designed around a single point worst load case Design Basis. The compression load is split between the SJAE stage and the LRVP stage. The interstage pressure determines how the load is split. Splitting of the load can be optimized to greatly reduce parasitic load, this is not addressed in this paper.

Figure 6 below shows a P&ID of a typical single train NCG removal system. At the design basis operating condi-tion the flow rate of the first stage and the second stage is balanced to maintain the design intercondenser pressure without any manipulation.

At off design conditions the load will not be balanced. The SJAE has a near constant mass flow capacity, while the LRVP has a near constant volu-metric flow capacity. As volume of a gas is dependent on the temperature and pressure, the volumetric load will vary for a set mass load. In addition water carryover from the intercon-denser to the second stage LRVP is dependent on temperature.

The 2nd stage LRVP will control the interstage pressure by either removing NCG volume at a rate faster or slower than it is added by the SJAE. Removing NCG faster than the first stage supplies will cause the interstage pressure to drop; conversely, removing the NCG slower than the first stage supplies will cause the interstage pressure to rise. In all conditions the system will either stabilize at the off design conditions naturally, with controlled assistance, or cease to operate properly.

Design Basis Operation

The following is the minimum information required for the design basis of the hybrid NCG removal system• First stage suction pressure (condenser design operating pressure minus the pressure drop in the vapor piping to

the first stage ejector)• Vapor temperature at the first stage suction• NCG gas mass flow rate• NCG composition or average molecular weight• Mass flow rate of air release from the cooling water and air in-leakage• Mass flow of water vapor to saturate the NCG and air at the condenser operating pressure and vapor discharge

temperature• Motive steam pressure available for the steam jet(s)• NCG percent weight content of the motive steam• Cooling water supply temperature• Discharge pressure of the NCG removal system

A typical 50mW (net) flash cycle geothermal power plant typically requires approximately 349,200 kg/hr of flash steam to the turbine. We will use this value with 3% NCG in the steam and the below design basis as our example.

Figure 6. Typical Hybrid NCG removal system.

Plant Net Power 50 mWSteam Flow 349,200 kg/hrNCG in the motive steam 3%NCG average MW 42First stage suction pressure 57.50 mbaraVapor Temperature 18.3 °CMass Flow of air neglectedMass Flow of Water Vapor Per HEI saturation formulaCooling Water Temperature 16.7 °C

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Choosing a 264mbara intercondenser pressure this system operates at the design basis as shown in Figure 7 P&ID.

Off Design Operation

A variety of differ-ent of design conditions can occur. Addressed be-low are various common scenarios. For all of these conditions the following general assumptions are made:

1. T h e m o t i v e steam usage by the SJAE have been estimated and not rigor-ously calculated by any SJAE manufacture.

2. Off design SJAE capacities have been estimated and not rigorously calculated by any SJAE manufacturer. 3. Vapor temperature at the condenser exit is a constant 0.9°C above the cooling water temperature.4. Condenser operating pressure is the vapor pressure at 9.3°C above the cooling water temperature.5. Intercondenser vapor approach temperature is 2.2°C6. Changes in heat load to the inter condenser are neglected.

Restrictions

If the compression ratio of the jets is pushed beyond the operating range the jets will become unstable.The LRVP uses cooling water as a sealing liquid. This water must remain within a margin of safety below the vapor

temperature of the LRVP suction. To facilitate this requirement a minimum interstage pressure is determined during the design. A valve is installed between the LRVP vapor discharge and inlet. If the interstage pressure falls below the minimum the recirculation valve will open and supplement the NCG load from the intercondenser, raising the interstage pressure. Minimum interstage pressure has been set at 225mbar.

Cooling Water Temperature Exceeds Design BasisThe cooling water provided for the Condenser, the intercondenser and the LRVP are all provided from the same

source, when this source temperature varies from design the temperature of water for each will vary the same amount. Condenser vapor temperature and condenser operating pressure need to be set for all conditions. For our theoreti-

cal condenser vapor temperature is assumed to be a constant 1.6°C above the cooling water temperature. Condenser Operating Pressure (unless restricted by the NCG removal system) is assumed the vapor pressure at 16.7°C above the cooling water temperature. This method can be used to study off design conditions of a particular plant with collaboration of the condenser and SJAE manufacturers.

The SJAE is a mass flow device with a (nearly) constant mass flow for a given compression ratio. As the condenser pressure is increased above design basis by the warmer than design cooling water, the intercon-denser pressure can also rise to maintain design SJAE compression ratio. Figure 8 below shows the required LRVP dry air equivalent volumetric flow rate that is required to maintain the design mass flow from the intercondenser at higher than design intercondenser pressures. (This curve is the NCG removal requirement

Figure 7. Design basis operation.

19.4°C

22.2°C

25.0°C27.8°C

30.6°C33.3°C

Design Basis,16.7°C

LRVP Capacity

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LRVP Suction Pressure, mbaraFigure 8. Dry air equivalent of volume to be removed from intercon-denser & LRVP Capacity.

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to maintain SJAE design basis compression ratio and does not represent any physical operating condition as the interstage pressure is set by the 2nd stage).

Figure 8 also shows the LRVP dry air equivalent capacity curve. At the design point the required volumetric capac-ity matches the LRVP capacity and the hybrid system will behave in a steady state, removing the design NCG load. Yet, the off design conditions do not lie on the LRVP dry air capacity curve.

At conditions of cooling water temperature exceeding the design temperature the volumetric capacity required to be removed from the intercondenser is less than the capacity of the pump. Since the LRVP is a positive displacement device it will remove NCG from the intercondenser faster than it is supplied by the SJAE. This evacuation will cause the pressure in the intercondenser to drop below the interstage pressure that corresponds to the SJAE design compression ratio, until the NCG flow volume meets the pump capacity as shown in Figure 9.

The intercondenser pressure (set by the LRVP) is now higher than design, the condenser pressure (set by the cooling water) is also higher than design. For all these cases the compression ratio of the steam driven air ejector is lowered. A typical capacity vs compression ratio SJAE curve is shown previously in Figure 3. As the compression ratio is raised above design the mass flow of the SJAE decrease and as the compression ratio drops below design the mass flow of the SJAE increases.

Therefore at these higher than design cooling water temperature conditions the capacity of the SJAE is higher than design, and the SJAE will displace additional water vapor above design bases into the intercondenser. This additional water vapor will add heat of condensation into the intercondenser and raise the discharge water and pos-sibly the gas outlet temperature. For the purpose of this example the possibility of the intercondenser approach temperature changing with the additional water vapor load has been neglected.

The result of the higher than design cooling water temperature is that the condenser pressure rises due to cooling water temperature and the intercondenser pressure rises, compression ratio of the jets is lowered from design, the compression ratio of the LRVP is raised from design (however the volumetric load to the pump is smaller than design basis). The NCG removal system does not limit condenser pressure. Once the SJAE reaches the end of its compression range, the system will become unstable.

Cooling Water Temperature is Less Than Design Basis

With cooling water below the design basis tempera-ture, the volume of NCG required to be removed from the intercondenser surpasses the LRVP capacity, as shown in Figure 10, and the pressure in the intercondenser will rise due to the NCG buildup until the pressure in the intercon-denser is high enough for the volumetric requirement to match a point on the LRVP curve. A minimum design inter-stage pressure will be set during design. Once this minimum pressure is reached volumetric load will be recirculated from the LRVP exhaust to the inlet maintaining minimum suction.

After the minimum interstage pressure has been reached, the compression ratio of the SJAE continues to try to increase as the cooling water temperature (and condenser pressure) lowers. Referring again to Figure 3 we see that the capacity of the jets decreases as the compression ratio exceeds design.

Condenser pressure is limited to the SJAE design compression ratio and therefore limited by the minimum interstage operating pressure at the design NCG load. Estimated condenser pressure is shown in Figure 11.

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LRVP Suction Pressure, mbaraLRVP Dry Air CapacityEstimated NCG Removal Capacity Requirements (at design jet CR)Design BasisEstimated Operating ConditionsMinimum Design

Figure 10. 2nd Stage Dry Air Equivalent Requirement, LRVP Capacity and Estimated Operating.

Figure 9. 2nd Stage Dry Air Equivalent Requirement, LRVP Capacity and Estimated Operating.

Design Basis

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Power requirements of the pump may vary slightly at off design conditions. Variation will vary depending on pump model. In this case the higher pump suction pressure the higher the power consumption. Variation in this example is less than +/-5kw.

Non Condensable Gas Load is Less Than Design BasisThe non-condensable portion of the steam flow is sometimes significantly reduced below design basis. A SJAE is a mass flow device with a (nearly) constant flow for a given compression ratio. Assuming that the jet is

allowed to operate at a fixed compression ratio at the condenser design pressure the required volumetric capacity, in m3/hr of dry air, that is nessasary for the second stage to remove is shown in Figure 12. (this curve does not represent any physical operating condition as the interstage pressure is set by the 2nd stage).

Since the NCG removal requirement of the SJAE has been reduced but the compression remains the same, the load to the SJAE is lower than design, and the SJAE will displace additional water vapor above design basis into the intercon-denser. This additional water vapor will add heat of condensation into the intercondenser and raise the discharge water and possibly the gas outlet temperature. For the purpose of this the possibility of the intercondenser approach temperature changing with the additional water vapor load, has been ignored.

With the mass of NCG required to be removed from the intercondenser reduced from the design basis, the volumet-ric requirement at the design pressure is also reduced. Since, the LRVP is a positive displacement device it will remove vapor from the intercondenser faster than required. This evacuation will cause the pressure in the intercondenser to drop until the required capacity meets the pump capacity, also shown in Figure 12. However, a minimum interstage pressure should be set during the de-sign process to prevent cavitation in the LRVP. When the minimum inter-condenser pressure is reached load to the LRVP will be supplemented from the LRVP discharge via a recirculation valve stopping the intercondenser pres-sure from dropping farther. Depending on the % of original load, the LRVP will operate somewhere between the design and the minimum interstage pressure.

As the condenser pressure is set by the cooling water temperature, there is no change in the condenser pressure.

Power requirements of the pump may vary slightly at off design condi-tions. Variation will vary depending on pump model. In this case the minimum suction pressure condition requires proximity +/-2kw from the design basis condition.

Non Condensable Gas Load is Greater Than Design Basis

With NCG load above the design basis, the NCG removal system will not be able to remove all of the NCG at the design condenser pressure. The condenser pressure will rise until the NCG removal system can evacuate the required gas. Analysis of off design op-eration in this case will have to be done with collaboration of the condenser manufacturer.

97.500%95.00%92.50%90.00%

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LRVP Suction Pressure, mbaraEstimated NCG Removal Capacity Requirements (at design jet CR)Design BasisEstimated Operating ConditionsLRVP Capacity

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LRVP Suction Pressure, mbaraLRVP Dry Air CapacityEstimated NCG Removal Capacity Requirements (at design jet CR)Design Basis90% NCG Removal Capacity Requirements (at design jet CR)80% NCG Removal Capacity Requirements (at design jet CR)

Figure 12. 2nd Stage Dry Air Equivalent Requirement, LRVP Capacity and Estimated Operating.

Figure 13. 2nd stage Dry Air Equivalent Requirement.

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Varying Cooling Water Temperature With Lower Than Design NCG Content Condition

The non-condensable portion of the steam flow is reduced below design while temperature of the cooling water provided for the Condenser, LRVP and intercondenser varies. For this example we consider the NCG load reduced to 90% and 80% of design.

Assuming that the jet is allowed to operate at a fixed compression ratio the

required volumetric capacity, in M 3

hrof dry

air, that is nessasary for the second stage to remove is shown in Figure 13. (This curve does not represent any physical operating condition as the interstage pressure is set by the 2nd stage) This curve is shifted down from the 100% NCG load condition.

At all conditions the NCG load is less than design basis and additional water vapor may be moved by the SJAE to account for the mass flow difference. This additional water vapor will add heat of condensation into the intercondenser and raise the discharge water and pos-sibly the gas outlet temperature. For the purpose of this example the possibility of the intercondenser approach temperature changing with the additional water vapor load, has been ignored.

Operating conditions will be simi-lar to the varying cooling water design, however, because of the lower flow rates the temperature that the intercondenser reaches the minimum design pressure is reduced. See Figure 14. Also, due to the decreased NCG mass flow to the SJAE, it is possible for the system to operate at a slightly higher compression ratio than design. Other lower NCG flow rates will be similar.

An important observation to make is that the condenser pressure at off de-sign conditions is limited primarily by the condenser cooling water temperature and secondly the compression ratio of the jets. As the required capacity of the SJAE(s) is lowered from the design ba-sis, the compression capacity increases, allowing very slightly lower condenser pressures than possible at design NCG loads, as shown in Figure 15.

Figure 16 shows the amount of excess capacity in each compression stage at 80% NCG condition.

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LRVP Capacity is satisfied byrecirculation

SJAE Capacity is satisfied by displacing aditional Water Vapor

Figure 14. 2nd Stage Dry Air Equivalent Requirement, LRVP Capacity and Estimated Operating.

Figure 15. Expected Condenser Operating Pressure.

Figure 16. 80% NCG, NCG capacity required each stage.

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Summary and Conclusions

1. In a system with a properly chosen design basis, the NCG removal system will not limit the condenser pres-sure until the minimum interstage pressure is reached.

2. The design basis must be the worst case scenario. The system will not operate effectively with higher than design basis NCG flow rate, and the condenser pressure will be limited.

3. Higher than design cooling water temperature with design NCG flow will cause:a) Condenser pressure to rise due to cooling water temperatureb) Intercondenser pressure will rise due to cooling water temperature and LRVP volumetric capacityc) SJAE will operate at a lower CR than design and displace additional water vapor and heat load to the

intercondenser4. Lower than design cooling water temperature with design NCG flow will cause:

a) Condenser pressure will attempt to drop due to cooling water temperatureb) Intercondenser pressure will remain between design and the minimum set pressure, at the minimum pres-

sure vapor will recirculate from the LRVP discharge to the inletc) Condenser pressure will be limited by the SJAE compression ratio

5. Higher than design NCG content will cause the condenser pressure to rise as it cannot be evacuated at a sufficient rate.

6. Lower than design NCG content will cause a) The intercondenser pressure to be between the design and the minimum pressure. b) Since the condenser pressure is set by the cooling water temperature there is no change.

7. Lower than design NCG flow rate with lower than design cooling water temperature will cause:a) Condenser pressure will attempt to drop due to cooling water temperatureb) Intercondenser pressure will remain between design and the minimum set pressure; at the minimum pres-

sure vapor will recirculate from the LRVP discharge to the inletc) Condenser pressure will be limited by the SJAE compression ratio. However as the NCG flow rate decreases

the jet can compress to a higher compression ratio and the condenser pressure lowers.

Notes

Note 1: The example geothermal power plant design used in this study does not represent a specific power plant, but rather is derived from many power plant designs with conditions averaged and normalized to a design capacity of 50mW net electric generation capacity.

Note 2: The example geothermal power plant design is based on 50mW net generating capacity, at the design basis using 97kg/s of geothermal steam with an average molecular weight of 43.5. Condenser operating pressure of .057bara. Cooling water is available at 16.7°C. Discharge pressure of the system is 1.034 bara. While the method of analysis remains the same, any change to any of the design basis parameters will affect the study.