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Molten Salt Pump Development for Concentrating Solar Power

Jan 18, 2018

There is an ever increasing world-wide demand for sources of energy. With only a limited supply of fossil fuels, and with the costs to obtain and produce those fuels increasing, sources of renewable energy must be found. Currently, capturing the sun’s energy is expensive compared to heritage fossil fuel energy production. However, there are government requirements on Industry to increase the amount of energy generated from renewable resources. The objective of this project is to design, build and test a long-shafted, molten salt pump. This is the type of pump necessary for a molten salt thermal storage system in a commercial-scale solar trough plant. This project is under the Department of Energy (DOE) Solar Energy Technologies Program, managed by the Office of Energy Efficiency and Renewable Energy.

There are some factors which need to be considered in the design of any molten salt pump. Some of the more significant factors are listed below:

Salt freeze up:
WARNING: This is a major problem. This type of pump has a very high risk of freezing up, due to the design of the hydro seal area. Several things will/can affect this, including (1) the specific labyrinth seal design and effectiveness of the nitrogen purge, (2) effectiveness at controlling leakage of salt into the tube from the bearing and threaded joint area, (3) consideration of the operational sequence of the pumps, (4) ensuring the seal can handle pump operation at shut off.

Salt leakage at discharge flange due to poor mounting plate design:
Molten salt is very difficult to handle. The slightest separation can cause it to leak through the ring joint flanges. Mounting plate design is critical.

Grease lubricated thrust bearings:
A grease lubricated thrust bearing is a preference. Oil lubrication can be a problem if leakage occurs.

Load carrying of thrust bearings:
WARNING: This is a major problem. The design of the thrust bearing carrier, and how the assembly is mounted, are very critical. This is the first point of failure for this equipment. Monitoring these bearings is critical. The bearings must be mounted and lubricated properly. This assembly needs one true thrust bearing and at least one radial bearing.

Sealing of thrust bearings:
Since oil lubrication is being used in a vertical arrangement, sealing is very difficult. This design has a very high risk of being overfilled, which can cause a fire. This type of lubrication system requires high maintenance.

Heat dissipation to the thrust bearings:
A thermal analysis must be performed to determine the heat dissipation through several of the major components of the pump, including the shaft, sump cover and pedestal. An accurate model of the thermal environment of the thrust bearings is critical in determining thrust bearing life. The bearing will see higher wear due to the long distance between the thrust bearing and the pump itself. In addition, high shaft runout will cause higher wear of the bearing, allowing additional molten salt to go into the tube (with its associated problems).

Hydro seal bearing clearance:
The bearing will see higher wear due to the long distance between the thrust bearing and pump itself. In addition, high shaft runout will cause higher wear of the bearing, allowing additional molten salt to go into the tube (with its associated problems).

Hydro seal vents:
Several different types of problems can occur depending on how much pressure and volumetric flow that the two vents see.

Axial adjustment:
CAUTION: Limited axial adjustment can be a major problem during the charging of the system with hot salt, and when replacing a rebuilt pump into a hot tank. Well defined procedures must be followed to prevent pump damage.

Structural design of the upper column flange:
WARNING: This is a major problem area. Due to the length of these pumps and the high temperature environment, design of the attachment point of the lower sections of the pump is critical. These pumps will see forces which will include sway, orbital, torsional and directional movement from currents inside the tanks caused by agitators, re-circulation systems and return lines.

Vibration due to long center between thrust bearing and first radial bearing:
The distance between the thrust bearing and the first radial bearing is very critical. Major problems from vibration can cause pump damage, requiring the removal of the pump and a major rebuild.

Leakage in threads joining hydro seal tubes:
Molten salt will wick through the threads joining the hydro tubes. The pressure of the discharge, as well as the joint location and the amount of salt wicking past the threads can allow salt to climb the shaft.

Shaft coupling design considerations:
WARNING: The intermediate shafts must be locked together to prevent them from unscrewing when a back flow occurs in the system. If not locked, catastrophic damage can occur.

Maintenance of pumps:
All rotating equipment will require maintenance. Equipment that has been in molten salt can be very difficult to disassemble. Designs that account for this, and allow for easy replacement of radial bearings, impeller, etc. will reduce overall costs for the customer more than would the purchase of less expensive pumps without this consideration.

Monitoring of radial bearings:
Monitoring of bearing health is critical. Monitoring will help to prevent catastrophic failures. In addition, it will allow accurate maintenance schedules to be developed, which will reduce lost time and increase profits.

Testing of long design in molten salt (or other molten fluids):
In-field testing, or laboratory testing with molten salt (or other molten fluids) is necessary to ensure satisfactory operation of these critical pumps. The long length of these pumps adds a new risk that must be evaluated.

Allowing pump to operate at shut off:
WARNING: Need to ensure the pump can be shut down for short periods of time without salt freeze up occurring. If internal salt freeze up were to occur, significant damage may occur to the pump during system restart.

Shaft/column sealing:
This is a critical area. The seal design must not be affected by the pump discharge pressure, otherwise salt may leak or spray out.

Lubrication of radial bearing:
It is critical that the design provide complete film lubrication for the bearings. Bearings that experience high pressure and direct discharge flow will wear faster.

Based on our findings, a commercial pump could be used for this application. This is providing that additional studies and testing of a full-scale pump have been completed, to ensure that the design modifications made to address the technical barriers were successful.

Molten Salt Pump Vertical Molten Salt Pump.pdf

Technical dept.
Hebei Tobee Pump Co.,Limited

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