Most expensive problems on a process project can be traced back to a drawing. A line that was never on the P&ID turns up during construction. A HAZOP stalls because the team is arguing about what the diagram shows. An instrument is procured to the wrong specification because the P&ID and the instrument schedule disagree. None of these are construction problems. They are drawing problems allowed to travel downstream.

The P&ID is the single document that everything else is built from. HAZOP works off it, procurement works off it, construction installs from it, commissioning checks against it, and the safety case relies on it being true. A weak one does not cause a single problem, it adds cost at every later stage, and it keeps costing after handover because operations and maintenance are still using it years later. We produce process design documentation with that whole life in mind.

What the work involves

Starting from an agreed design basis. Before drawings, we set out the process design basis: flow rates, operating and design pressures and temperatures, fluid properties, heat and mass transfer duties, utility demands and any environmental or regulatory constraints. Agreeing this early means the whole team designs against the same numbers. From it we carry out the sizing work – pipe and pump sizing, pressure drop, heat exchanger duty, separation and reactor parameters.

PFDs before P&IDs. Process flow diagrams establish the concept in a form your operations and management teams can review without wading through detail: major equipment, main process and utility streams, stream data and high level material balances. It is far cheaper to challenge a concept here than on a P&ID.

Developing the P&ID. We then build the P&ID out to the detail construction and commissioning genuinely need: every process and utility line with number, size and specification, valves, strainers, vents and drains, tagged equipment, instrumentation, and the control loops and interlocks that define how the plant behaves normally and when something goes wrong. Consistency with line lists, valve lists and instrument schedules is maintained as we go, under revision control, so the documents do not drift apart between issues.

Control philosophy and relief. Alongside the diagrams we write the control philosophy – modes of operation, start up and shutdown sequences, key control loops, alarms, interlocks and emergency shutdown responses – giving your system integrator something unambiguous to program against. We also identify overpressure scenarios, size relief devices to API 520 and API 521, and design the discharge routing. Where a safety instrumented function is needed, that work follows BS EN 61511.

What you get

  • An approved design basis with assumptions and data sources recorded
  • Process flow diagrams with stream data and material balances
  • Fully developed P&IDs, revision controlled and consistent with the supporting registers
  • Line lists, valve lists and instrument schedules that match the drawings
  • Process calculations covering sizing, pressure drop, thermal duty and utility demand
  • A control philosophy covering modes, sequences, alarms, interlocks and shutdown responses
  • Relief scenario assessment, device sizing and discharge routing, documented for review
  • Drawings issued fit for HAZOP, procurement, construction and handover

When clients typically call us

  • A HAZOP is booked and the P&IDs are not in a fit state to be reviewed
  • The drawings no longer match the plant after years of small modifications
  • A capital project needs a design basis and FEED level package before investment approval
  • A set of P&IDs has been inherited with a site or acquisition and needs verifying
  • A relief system has never been formally assessed, or conditions have changed since it was
  • A control system upgrade is planned and there is no written control philosophy to work from

Frequently asked questions

Can you work from our existing drawings?

Yes. Much of our work is bringing an existing set up to date rather than drawing from scratch. We survey what is installed, mark up the differences and reissue under revision control.

Will you use our drawing standards and title blocks?

Yes. Where you have a house standard, symbol set or CAD system, we work in it. Where you do not, we use recognised conventions and agree the approach with you first.

Do you chair the HAZOP as well as produce the P&IDs?

We can do both, though many clients prefer an independent chair. Either way we attend, respond to actions and carry the outcomes back into the drawings rather than leaving them in a spreadsheet.

How does the design basis affect cost?

It is the main defence against under design and over design. Agreeing operating envelopes and margins up front avoids rework on an undersized system and capital wasted on an oversized one.

Related: Part of our engineering design services. See also pressure relief system design, HAZOP facilitation and 3D laser scanning and modelling.

If your P&IDs are the weak link between a sound process idea and a plant that gets built correctly, that is worth fixing early. Talk to us about your plant. Request a call