Heat and Material Balance
Heat and Material Balance (H&MB) is one of the basic process engineering documents produced by process design engineers in which energy and material output from a system equated energy and material input. It includes operating conditions, compositions, and key physical properties of every major process stream on the Process Flow Diagram (PFD). A H&MB may be included as part of a PFD or a separate document, particularly when several operating cases are evaluated.
Normally a heat and mass balance sheet reports following data for a process stream:
Normal operating temperature and pressure.
Normal volumetric or mass flowrate. If multiple phases are involved, flowrate for each phase should be reported.
Density at normal operating temperature and pressure conditions. If the stream has multiple phases, density for every phase should be reported along with the overall density.
Viscosity for each phase in the stream should be separately reported.
If gases are present, vapor fraction should be reported.
Specific heat ratio Cp/Cv and compressibility factor should be reported for gaseous phase.
Molecular weight for each should be reported separately.
Enthalpy flow for each stream is also reported sometimes in KJ/hr.
Heat and mass balance calculations for a process are performed by applying the mass balance equation and the energy balance equation to each equipment. This provides the solver with a set of equations. Above mentioned properties of each stream are the unknown variables.
Typically, softwares such as Aspen Hysys and FlowBal are used to generate H&MB reports.
Line and Equipment Lists
A line list (or line schedule) is a database of all process lines in a facility design as shown in the Process and Instrumentation Diagram (P&ID). It is created to communicate between the process and mechanical or piping engineering teams when designing piping in a plant or process unit. At its core, it should help provide justification on why certain decisions were made in the design process.
Uses of Line Lists
For Process Engineers – It specifies the process requirements such as the design and operating pressures and temperatures, flowing medium, piping code, Line Identification, reference P&ID numbers etc.
For Mechanical / Piping Engineers – It contains line numbers, connections, isometric numbers, and other items that are related to the overall construction of the piping. It is used to ensure that the lines on the P&ID are accurately reflected in the piping design.
Content of a Line List
At a minimum, the information on a line list is as follows:
Line identifier (line designation, line number) – a unique number that identifies the line in the plant or process unit.
Start and ending point of the line – includes equipment connections or connections to other lines.
Service or commodity – materials that are flowing through the piping
Fluid phase – liquid, vapor, two phase, etc.
Piping specification and/ or piping code
Design and operating pressure and temperature of the service
P&ID reference numbers
Piping Isometric reference number
Corrosion allowance
Insulation type and thickness
Heat tracing type
Special calculation requirements such as pipe stress
Special Information & requirements such as post weld heat treatment, pickling, hydrotest requirements, etc.
An Equipment List is a listing of all tagged equipment with their various details. The Equipment List is initiated and developed by the process team while the mechanical engineering team updates and finalizes it.
Content of an Equipment List
• Equipment Name
• Equipment number
• Service description
• Capacity
• Dimension and size
• Weight
• Required power
• Purchase Order (PO) number
• Reference P&ID numbers
• Key summary information of all tagged equipment items.
SAFE Chart
Oil and gas production platforms integrate automatic safety systems to react to equipment failures. A Safety Analysis Function Evaluation (SAFE) chart is a matrix used to design these safety systems. It shows what can go wrong with equipment and what happens to shut it down safely. SAFE charts provide a mechanism for considering every component in the facility and then, for each component, fully account for each required safety device. SAFE charts are used to ensure that the facility is as fully protected as it should be and can be used as a troubleshooting tool.
SAFE charts have been around since the introduction several decades ago of the American Petroleum Institute (API) standard API RP 14C: Recommended Practice for Analysis, Design, Installation, and Testing of Safety Systems for Offshore Production Facilities.
PSV (Pressure Safety Valve) Sizing
No chemical process facility is immune to the risk of overpressure. For every situation that demands safe containment of process gas, it becomes an obligation for engineers to equally provide avenues for pressure relieving and flaring wherever necessary. The levels of protection are hierarchical, starting with designing an inherently safe process to avoid overpressure followed by providing alarms for operators to intervene and Emergency Shutdown provisions. Beyond this design and instrument-based protection measures, the philosophy of containment and abatement steps such as pressure relieving devices is employed. Pressure Safety Valves are one of such devices.
Pressure safety valves (PSVs) are designed to protect personnel and plant properties from an overpressure occurrence in equipment or piping by relieving fluid to a safe location. While other limitations exist, such as material selection and temperature-pressure rating, a proper sizing can guarantee and optimize protection. The first (and potentially most) important step in the design of a safety valve is to size it as accurately as possible.
Properly sized relief valves will provide the required protection, while also avoiding issues with excessive flow rates, including possible valve damage, impaired performance, undersized discharge piping and effluent handling systems, and higher costs. Many scenarios can result in an increased vessel pressure, and each scenario may result in a different valve size. Typical scenarios or cases include:
Blocked Liquid Discharge Case
This refers to closure of a valve on the outlet of an equipment. With continuing liquid flow into the equipment and no provision to drain the liquid, fluid accumulates, building pressure to as high as the design pressure of the upstream equipment. Static head of the liquid in the upstream equipment also contributes to the buildup of pressure.
Blocked Gas Outlet (Non-Fire Case)
Like the above case of liquid, gas accumulation in the vessel also contributes to the rise in pressure when the gas side valve fails to function by staying closed. With pressure continuing to rise, a relief device is required to relieve the equipment of the excess pressure.
Gas Control Valve Fail Open
This case refers to a scenario where when a control valve placed between equipment (where the upstream equipment has a higher design pressure than the downstream equipment) fails open, causing over-pressurization.
Thermal Expansion
This case occurs when liquid gets locked inside liquid lines. With exposure to sunlight and heat, temperature rises and vaporizes the liquid, resulting in over-pressure.
Fire Case (Liquid Filled Vessel]
All equipment in a process facility are prone to exposure to fire due to equipment failure or human errors. This can result in the fluid content of the equipment expanding and vaporizing to create an over-pressure scenario. Fire cases are of two types – Gas Filled Vessel and Liquid Filled Vessel.
PSV Sizing Basics
In principle, PSV sizing should follow the general steps below:
Design engineer to identify and estimate the applicable PSV relief scenarios based on cases in API 521 RP or other detail heat transfer model. This is a key step as some of the scenarios require inputs from people with great knowledge on the process design and unit operation. Each of the scenarios need to be carefully evaluated to avoid under sizing the PSV.
Next, an appropriate PSV type must be selected, as sizing depends on the type of relief device selected. Below are a few PSV types:
Conventional Safety Relief Valve – It has a spring housing that vents fluids to the discharge side of the PSV. The operational characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by changes in PSV back pressure.
Balanced Bellow Safety Relief Valve – A balanced safety relief valve provides a set of bellows to reduce the effect of back pressure on the operational characteristics.
Pilot Operated Safety Relief Valve – In a pilot operated safety relief valve the major relieving device has a self-actuated auxiliary pressure relief valve to control the relieving conditions.
Power Actuated Safety Relief Valve – In a power actuated safety relief valve, the major relieving device is controlled with an external source of energy.
Temperature Actuated Safety Relief Valve – In a temperature-actuated safety relief valve, the actuation takes place by external or internal temperature or by inlet side pressure.
Pressure Vacuum Safety Relief Valve – A vacuum relief valve is designed prevent excessive internal vacuum and further fluid flow after normal conditions have been restored.
Next step is to calculate the thermal and physical properties of the fluid (gas / liquid) at relief temperature and pressure. The latent vaporization heat would be key for the PSV sizing calculation in fire case. This calculation is normally aided using a commercial process simulation software (ASPEN HYSYS, ProII etc.).
Then, calculate the required orifice size as per defined project standards.
Finally, select the PSV size.
Process Datasheets
Process Simulations
