A Single Failure Proof (SFP) crane has redundant systems and mechanisms built into it as a measure to prevent untoward incidents. This backup system ensures that the work to be performed is completed, even in the remotest possibility of any one of the system or mechanism fails. In the case of these cranes, it means that if a load carrying system suffers a failure, the full load is taken by the backup or redundant mechanism. ElectroMech has provided Nuclear Power Corporation of India (NPCIL) with three SFP cranes, each achieving the SFP criteria through a different solution.
In the two 60t cranes, the required redundancy is achieved by using two independent load paths through two rope drums. The 75t crane has a unique three hook arrangement to a yoke, which includes a main hook of 75t and two 40t hooks on either side.
Quality Assurance Plan delivering maximum satisfaction
NPCIL and the quality team at ElectroMech worked in close integration with each other to ensure that the cranes meet the required safety parameters. A detailed Quality Assurance Plan (QAP) was devised to follow the quality standards step by step at every stage of the crane development, keeping in mind that zero error is achieved. The QAP followed a five stage process, which can be elaborated as follows:
Defining, drafting and finalising of the quality plan and procedures with respect to the requirements of NPCIL, and obtaining the approvals on these procedures & schedules from NPCIL’s design and quality team.
A well-documented/detailed material identification process was followed along with stamp transfers of materials from NPCIL for all the raw materials and components. The Non Destructive Examination (NDE) and testing of all material and components were observed by the NPCIL team at NABL accredited labs.
Several tests were conducted for each component to verify it as per quality parameters.
Yoke and hook load and overload testing was conducted by the ElectroMech team in presence of NPCIL’s quality and design team at a special facility.
Full crane functional testing for load and overload test was observed by NPCIL for various conditions and parameters.
The handling challenges at NPCIL: Crane 1
One of the biggest challenges while designing this crane was that the SFP crane for the 700MW reactor was to be made with the same dimensional constraints as a regular EOT crane. Moreover, the span of the crane for the 700MW reactor is 21m as compared to 10m for a crane in the earlier 500MW reactors.
Design to ensure structural integrity during a seismic event was called for. The crane structural needed to be designed for Safe Shutdown Earthquake (SSE) and Operating Base Earthquake (OBE) standards as per the furnished Floor Response Spectra. For the design and qualification of the plant the SSE standard is adopted. The intensity of the OBE is related to the production period of the nuclear power plant. After an OBE, it should be possible to shut down the plant safely and further operation of the plant.
The Floor Response Spectra is tabulated data that gives the acceleration forces that may act on the crane in the event of an earthquake. These acceleration values are a function of equipment frequency and the elevation, amongst many other factors. By using the data provided by NPCIL for the particular site location, the crane structure was suitably designed & qualified.
Procurement, quality and production
The special design called for use of special/non-standard materials. Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) were specifically established to meet the customer requirement as per American Society of Mechanical Engineers (ASME).
Solution from ElectroMech: Crane 1
ElectroMech design team, along with the NPCIL design team, conceptualised a double-decked arrangement of the hoisting trolley mechanism. The design of this trolley, with a height of approximately 6.5m and sitting on top of the reactor building, was successfully undertaken by ElectoMech design team. This new concept was endorsed by the NPCIL team.
The ElectroMech design was reviewed by the NPCIL design team and handed over to a third party for analysis using softwares to simulate virtual conditions. Finally, after detailed engineering, the structural design was approved.
After successful manufacturing, this was tested for full load and overload in ElectroMech factory before despatch in the presence of a specialist team from NPCIL, QS Pune team, which inspected the cranes for assuring quality and NPCIL Mumbai team, which provided support regarding the design inputs at several stages.
The handling challenges at NPCIL: Crane 2
Designing an SFP crane is always a challenging task. There were a few more additional constraints while designing this crane, such as its span being only 6.8m. This demanded the trolley including redundancy machinery, safety equipment, like caliper brakes and auxiliary machinery, to be tightly accommodated in a trolley area of 6.5m×5.5m.
This crane will be used to handle spent fuel rods in the Spent Fuel Storage Bay, where they will be stored in spent fuel pools. Spent fuel rods from the nuclear reactors are stored for 10 to 20 years in storage pools that are typically 40+ feet deep. These pools contain water, which acts as a shield to prevent radiation from entering the atmosphere as well as cools the fuel assemblies that continue to produce heat for some time after extraction of rods. The bottom is equipped with storage racks designed to hold fuel assemblies. Used nuclear waste, in the form of tiny pellets, is loaded onto metal rods that are then bundled into a ‘fuel assembly’. The assemblies are stored inside casements that are then submerged in cooling pools. The application of the 75/30t SFP crane supplied by ElectroMech will be able to handle these casements.
Solution from ElectroMech: Crane 2
The crane lower block has been provided with three hooks – a 75t main hook and two 40t hooks on either side. They are positioned in such a way that the composite lifting assembly meets the requirement of single failure criteria. During regular operations, the load will be lifted by the 75t hook and in case of failure of this hook, the load will be borne by the two x 40t hooks.
This lower block gets engaged with a three-pin arrangement to a specially designed yoke, which is a lifting attachment with four 25t hooks. This yoke will handle the spent fuel casements kept underwater. Stringent guidelines were specified by the customer considering the critical nature of the application. Some of the special requirements for the underwater application include galvanising the lower block and treating the yoke hooks with manganese phosphate. This coating treatment to the hooks enhances the overall life cycle, making it durable in extreme conditions.
Full load and overload testing of the crane (factory testing)
The crane was tested at 100% & 125% of the full load capacity. This was based on two conditions – first, with load & overload on the 75t centre hook and then again for the two x 40t hooks.
Testing of engagement & disengagement of the crane with yoke (factory testing)
The yoke is a critical component of this crane and its engagement & disengagement with the crane is of paramount importance. This was tested for repeatability and accuracy in the presence of the NPCIL technical team.
Overload testing of the yoke separately at 150%
As a critical link between the load and the crane, the yoke was required to be tested at 150% of the rated load. The testing was conducted by using a 200t mobile crawler crane. The test load was lifted by suspending the yoke to the crawler crane hook.
This crane was despatched after rigorous inspection and testing in the ElectroMech factory in the presence of NPCIL experts. They were extremely happy with the timely delivery and excellent performance during the various trials and tests taken at the shop.
How NPCIL benefited from ElectroMech solutions
Indigenous design & manufacturing and large manufacturing capability helped in shortening the procurement time and meet the project deadline
The innovative concept of two-tiered crab design ensured dimensional constraints, which were met even when there was an increase in the crane span and capacity
Assured competent service through Cranedge, ElectroMech’s service subsidiary
Courtesy: ElectroMech Material Handling Systems