PSV Plus - Process Software for Pressure Relief Valves Calculation and Design

PSV Plus Technical Information

CALCULATION CODES

Pressure Safety Valves calculations are carried out in full accordance with APT RP 520, 521
and 526. Good engineering practice criteria are also available in PSV Plus to help the
process engineer in the selecting proper Relief valve parameters and type and achieve a
correct sizing.

PSV PlusTM includes a set of routines for the Gas, Liquid, Steam and Two-Phase relieving
scenarios. The Two-Phase code provides four different calculation routines in accordance
with Appendix D of API 520 latest edition (i.e. DIERS method).

SIMULTANEOUS PROCESS DATA SHEET PREPARATION

Based on actual user specific requirement, the calculation modules can be linked to an
AccessTM (*) database, for the immediate preparation of the following documents, while
performing the orifice and in / out lines design:

Process Specification Sheets (i.e. the data sheets normally prepared by the Process
Engineer used by the Instrumentation Dept as basic information to purchase the
PSV’s);

Summary Calculation sheet for PSV : the calculation sheet that summarizes all the
scenario’s considered for each PSV sizing, along with the most relevant data. This
document is normally required by end users, authorities and certifying entities to
ensure that all applicable emergency scenario’s have been duly taken into
consideration;

Relief Load Summary: table normally used to show in a consistent format all the
relieving rates and characteristics for plant general emergencies. Required for sizing
and design of flare headers and sub-headers.

All the above reports will be generated automatically starting from the PSV calculation
stage.
The MS Access database has also been designed to be fully compatible with Smart Plant
Instrumentation (ex-INToolsTM) for automatic data download, if required.

FLUID PHYSICAL PROPERTIES

PSVPlus uses FluidPropTM software for the calculation of single-components and mixture
properties as defined by the user. More than 150 components and five different
thermodynamic models are available to estimate fluid properties at relieving conditions.
Fluid composition can be specified by the user and all the required properties for pure
compounds or mixtures automatically calculated at relieving conditions. Five different
thermo packages are available to properly calculate these properties depending on the
type of mixture to be considered.

INLET AND OUTLET LINES SIZING

Inlet and outlet lines sizing routines for single and two-phase flow patterns are also
included in the software for proper sizing after safety relief valve orifice has been calculated
and selected. PSV PlusTM will automatically check that the specified back pressure of the
valve is consistent with the one calculated across the outlet line and will alert the designer
in case of improper valve selection, based on the back pressure to upstream pressure ratio
criteria.

Inlet and outlet lines sizing is also checked against the pressure drop and velocity criteria
commonly used by engineers. The option of sizing the outlet line for sonic flow condition is
also available.

A calculation form is provided for easy and quick calculation of inlet / outlet piping
equivalent length, in case a detailed calculation is required by the user.

DETERMINATION OF RELIEVING RATES, ACCORDING TO THE APPLICABLE
EMERGENCY SCENARIO

The rate to be discharged from a PSV is normally to be calculated assuming all the
applicable emergency scenario’s.

This is a time consuming task that requires expertise and often heavy hand calculations.
PSV Plus provides a set of routines to quickly guide the user towards the proper
determination of this relieving rate, by suggesting typical parameters and procedures to be
considered, in addition to calculation methods:

Fire Load calculations

PSV PlusTM eliminates the need to perform any fire load related calculation.

The

options available

will allow to calculate fire loads for both Wetted and

Unwetted

vessel surface cases. The fire load

will then depend on selected

Vessel geometry, its

dimensions and elevation and fluid latent heat.

Up to three vessels can included as

contributors to the total fire load calculation.

C ontrol Valve Failure

A detailed calculation form is available to rigorously calculate the

consequences of

the complete

opening of a control valve. Calculation

routines take into account

control valve CV calculation, if not

already available, and will cover also the gas break-through situation, that is

one of

the most

frequent scenario encountered.

Fan Failure

In case of failure of one or more fans the loss of cooling or condensing may

lead to

pressure

increase in the system and subsequent opening of Pressure

Safety Valve.

Determination of the

relief load is then achieved through typical

parameters.

Reflux Failure

Failure of a column top or side reflux causes pressure increase in the system as

higher vapor rate

than normal will be present into the column. Relieving rate

determination is achieved through

typical parameters.

Exchanger Tube Rupture (Single and Mixed Phases)

When the differential pressure between the two sides of a S&T heat exchanger

high, the tube

rupture contingency is a main concern since, should it occur,

the low

pressure side may be affected

by pressure higher than design one. Determination of relieving load for this

case is

accomplished

through complex calculation routines, that are

available in PSV Plus.

Thermal Expansion

Should a liquid filled portion of piping or equipment be blocked-in under solar

radiation or any other

heat source, this will cause pressurization due to specific

volume change of the liquid. Calculation

of relieving rate can be easily

calculated

through PSV Plus routines.

General Power Failure, Cooling Water Failure, Fire, Blocked Outlet, etc…

The other emergency scenario’s can be selected from a comprehensive list.

However

no standard

calculation method exists for them and the Process

Engineer will have to

calculate on his / her own.

PSV Plus provide facilities to

record all the considerations

made to achieve the relieving rate figure,

for

later use or documenting purposes.

SEAMLESS INTEGRATION BETWEEN PSVPlus CALCULATION ENGINE AND PSVPlus
ACCESS DRIVEN DATABASE APPLICATION

Microsoft Access database environment can be accessed directly from within PSV Plus
calculation engine.

This database is distributed in the Open Source form, allowing the user to customize
reports, adding Company logo’s and performing any kind of modification required by the
end user.

All the PSV’s emergency scenario’s calculations are handled in PSV Plus calculation
modules. Immediately, without the need of any re-typing work, the required reports are
available in the linked database and can be exported to Microsoft Excel, if required.

ON-LINE GUIDE AND QUICK START TUTORIAL

PSVPlus is provided with an online help and a comprehensive quick start tutorial section
that will guide the user through main software capabilities.

One-hour self-training, using this tutorial, will be sufficient to an experienced professional
to achieve a satisfactory use of the software.

Technical reference and background is also provided in the manual.

MAN-HOURS REDUCTION AND JOB QUALITY IMPROVEMENT

PSVPlus has proved to be effective in saving man-hours up to 70% for the pressure safety
valves calculation and process specification / reporting activities, when compared to a
spreadsheet based activity.

Enhanced job quality is also provided by the standardization of calculation codes and full
data consistency among the generated reports.

Softbits Consultants Ltd.
info@psvplus.com, +44 1420 561 511, fax +44 87 0051 6362