What is the difference between basic process control system and ESD (emergency shutdown system) (PESS)?

The difference is the ESD has more safety features than the normal control system.
These features are measured by what is called SIL (Safety Integrity Level) that identifies where such system can be used.
Each area in which an ESD system will be used is classified by a certain SIL level from1 to4.1 being the easiest conditions and4 being the toughest conditions.
In most cases redundancy is mandatory in all levels for an ESD system with SIL3 level.

ESD is used for safety applications, it's all based on manufacturing techniques and specifications that will affect finally the MTF (mean time of failures) and the PFD (Probabilty of Failure on demands).

Higher SIL rating loops (3 and up) will most likely require an ESD system to perform it, while normal controlling loops (SIL1 &2) can be achieved easily by a normal process controllers.

Although the MTF and the PFD of a DCS has been decreased, and safety loops can be achieved on some DCS systems, designers still prefer to use special ESD systems.

 

I think that the gap between different systems has been decreased over time, and who knows may be one day it will vanishe.

Also ESD systems are used where Safety is the priority.
The field devices in critical processes will be redundant and the trip logics are complex.
The ESD architecture is based on SIS(Safety Instrumented System) which are implemented from HAZOP safety studies.
Safety is the priority and the main difference comparing both these systems.

he difference is the ESD has more safety features than the normal control system.

usingan ESD button you can close a full station at emergancy suitation

In the process plan protection layer, the basic process control system (BPCS) is part of second protection layer while ESD is the fourth layer of protection layer. Any upset condition of the process will be re-conditioned by BPCS. The BPCS includes operator, HMI alarms, monitoring system, etc within operator supervision. If the upset is escalated then safety instrumented system will take action to prevent the plant, man, machine and process from damage.. in case (which we dont want to) the upset is continued.. then mitigation steps shall be in-place... this is the fourth layer and ESD is part of it.

 

An Example... tank pressure is slightly above the set point.. then HMI alarm will be activated allows operator to take action.. however the operator is sleeping and the automatic function is also not functionin... then the pressure is tend to increase more and more then already out-of-control.. it means the BPCS have failed to protect. Next, there is pressure relief valve  shall open in such circumstance... but.. somehow it is not open... then the process shall be turned down by ESD to prevent damage.

you can find the detail expalanation of this.. on IEC61511 part3.

hope this helps.