In the COntrolled volume pumps - (as per API 675), the set pressures for the Internal and External safety valves are differently being followed by each vendor. Some vendor set the Internal set pressure more than the External RV set pressure and some vendors in the other way. Is this purely based on the design of the pump offered or based on any other factor?

I am basing my answer on a belief that you are describing an oil actuated diaphragm pump that has an internal relief valve on the oil system and an external relief valve on the process.  If so, the internal valve is set by the pump manufacturer to protect the pump.  It is there to prevent the excess oil pressure from bursting the diaphragm when the pump is operated with little or no back pressure.  The external relief valve is there to protect the piping and downstream process equipment.  It is possible for a given design to have a diaphragm burst pressure that is higher or lower than the pressure rating of the downstream equipment.  So, either one could be higher.  The pump manufacturer would not normally reduce their internal relief set pressure just because you are installing the pump in a system with a lower pressure limitation than their diaphragm.  Of course, it might seem that an external relief valve would be unnecessary if the internal relief valve is set to relieve at a lower pressure.  However, the internal valve is not a fully capable PSV that could be tested and certified.  So it would not satisfy the mechanical integrity requirements in most plants.  The external relief would still be needed to satisfy plant MI requirements, API or other process industry standards, etc.   

I am not sure how you would accomplish what you are proposing.  You cannot increase the set-point of the internal relief valve without risking a burst diaphragm.  You have the option of reducing your external relief setting if you choose.  You may have plant relief policies that dictate the maximum set-point based on the design capability of the downstream equipment. But, you may have discretion to reduce the set-point.  But I would not encourage this. The internal relief should not be counted as part of your protection. It has the advantage of being continuously tested by the operation of the pump.  But, it is not of a design and construction suitable for system protection.

Lastly, if the internal valve has a lower set-point, it is still providing some additional protection.  You external relief valve has to be tested according to some mechanical integrity program. Just because the external valve should never relieve does not mean it is not providing just as much protection.  I don't see any value in trying to coordinate the set-point of these two relief valves that exist for totally different purposes.

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