What Is a Battery Limit? Plant Boundaries

Quick Answer: A battery limit (BL) is an imaginary boundary that defines the physical and contractual extent of a process unit, plant, or EPC scope of work. Piping that crosses a battery limit connects the unit to external systems (feed supply, product export, utilities, flare). Battery limits determine where one party’s responsibility ends and another’s begins, and they are the reference points for defining tie-points, metering, isolation, and interface documentation.

Types of Battery Limits

Type	Definition	Example
Process battery limit	Boundary around a single process unit	The CDU (crude distillation unit) battery limit separates it from the VDU (vacuum distillation unit)
Plant battery limit	Boundary around an entire plant or facility	The refinery fence line separating the refinery from the crude oil terminal
Contractual battery limit	Boundary that defines the scope of an EPC contract	EPC-1 is responsible for everything inside the battery limit; EPC-2 handles the offsite piping
ISBL (Inside Battery Limits)	Area within the process unit boundary; includes all process equipment, piping, and instrumentation	Reactors, columns, exchangers, pumps, and associated piping
OSBL (Outside Battery Limits)	Area outside the process unit; includes utilities, storage, flare, and interconnecting piping	Tank farm, cooling water system, steam generation, pipe racks between units

Battery Limit Information on Engineering Documents

Document	How Battery Limit Appears
Plot plan	Dashed or dotted line around the unit/plant boundary; labeled “Battery Limit” or “BL”
P&ID	Battery limit flags on lines crossing the boundary; reference to connecting system or P&ID
Line list	”From” or “To” field references the battery limit (e.g., “To BL” or “From OSBL”)
Piping isometric	Isometric terminates at the battery limit with a tie-point reference
Piping plan	Battery limit shown as a boundary line with coordinate references

What Happens at the Battery Limit

Activity	Description
Isolation	Block valves (usually gate or ball) are installed at the battery limit to isolate the unit from external systems for maintenance, shutdown, or emergency
Metering	Flow meters, analyzers, or custody transfer instruments are installed at the battery limit for process control and commercial measurement
Spec break	The piping specification may change at the battery limit if the ISBL and OSBL piping are designed to different standards or materials
Tie-points	All piping connections across the battery limit are documented in a tie-point register with agreed coordinates, sizes, ratings, and flange details
Responsibility transfer	Engineering, procurement, construction, and testing responsibilities change at the battery limit between different contractors or project phases

ISBL vs. OSBL Comparison

Parameter	ISBL	OSBL
Scope	Process equipment and piping within the unit	Utilities, storage, interconnecting piping, flare
Pipe routing	Short runs between equipment; complex geometry with many fittings	Long runs on pipe racks; simpler geometry; larger diameters
Design code	Typically ASME B31.3 (process piping)	ASME B31.3 (utilities) or ASME B31.4/B31.8 (pipelines)
Cost estimation	Estimated per equipment item or per unit area	Estimated per linear meter of pipe rack or pipeline length
Contractor	Usually the main EPC contractor	May be a separate contractor or the same EPC under a different work package

Common Mistake

Failing to clearly define the battery limit in the early project phases (FEED stage). Ambiguous battery limits cause scope gaps where neither contractor takes responsibility for a section of piping, leading to late-stage change orders, schedule delays, and uncoordinated tie-points.

Battery limits are established during front-end engineering and are documented on the plot plan, P&IDs, and interface registers. They are a key reference for all piping engineering, procurement, and construction inspection activities.

Read the full guide to piping engineering