NeqSim’s Standard_ASTM_D6377 provides an equation-of-state screening calculation for vapor-pressure quantities associated with crude oil and condensate. The class name and method labels follow ASTM D6377 and historical ASTM D323 terminology, but the implementation is not the prescribed laboratory apparatus or compliance evidence. Use a qualified laboratory result and the applicable contract, regulation, and current controlled standard for custody transfer or product acceptance.

Quantities and Method Boundaries

The class calculates several distinct quantities at one configured reference temperature.

NeqSim result Current calculation Interpretation
TVP EOS bubble-point pressure Thermodynamic screening value for the supplied fluid model
VPCR4 Pressure from TVfractionFlash(0.8) after the bubble-point solve Raw vapor/liquid volume ratio 4:1 result, not an 80% molar vapor fraction
RVP_ASTM_D6377 0.834 * VPCR4 NeqSim D6377-labelled RVP-equivalent correlation
RVP_ASTM_D323_82 0.752 * VPCR4 + 0.0607 with pressures in bara NeqSim historical D323-labelled correlation
VPCR4_no_water VPCR4 calculation on a clone with water removed Water-free comparison, calculated lazily
RVP_ASTM_D323_73_79 Water-free VPCR4 result Implementation equivalence, not endorsement as a laboratory D323 procedure

These are model outputs, not interchangeable measurements. Report the selected method, reference temperature, pressure unit, fluid characterization, equation of state, and mixing rule with every result.

The default method is VPCR4. Select a method with the type-safe Standard_ASTM_D6377.RvpMethod enum and read it through getRvpResult(). The structured result contains the value in bara, method label, reference temperature in degrees Celsius, and a validity flag. Use getValue("RVP", unit) only for the currently selected method and getValue("TVP", unit) for the EOS bubble-point result. Do not call getValue("VPCR4", unit); that return-parameter name is not supported by the unit-aware legacy getter.

State Ownership and Model Selection

calculate() sets temperature and pressure and performs flashes on the SystemInterface supplied to the constructor. Pass a clone when the caller must preserve the original fluid state. This does not guarantee independence of every model parameter: binary interaction arrays can be shared across clones in some versions. Do not mutate BIPs through a clone. Use separately loaded or constructed fluids for parameter studies and verify that the source remains unchanged.

The standard does not force SRK or any other equation of state. It uses the supplied system’s thermodynamic model, composition, characterization, and mixing rule. Vapor-pressure predictions can be sensitive to light-end loss, heavy-end characterization, water handling, and binary interaction parameters. Validate the chosen fluid model against representative laboratory data.

The default reference temperature is 37.8 degrees Celsius. Changing it is supported by the API, but the result must then be reported at that configured temperature rather than presented as a 37.8 degrees Celsius standard result. The current isOnSpec() implementation always returns true; apply project or contract limits explicitly instead of using it as a compliance check.

Executable Java 8 Workflow

The program below preserves the source fluid, selects the D6377-labelled correlation, checks the structured result, reads the EOS bubble-point pressure, compares VPCR4 and its water-free variant, and converts the selected RVP value to kPa.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.standards.oilquality.Standard_ASTM_D6377;
import neqsim.standards.oilquality.Standard_ASTM_D6377.RvpMethod;
import neqsim.standards.oilquality.Standard_ASTM_D6377.RvpResult;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;

public final class AstmD6377Example {
  private static final Logger logger = LogManager.getLogger(AstmD6377Example.class);

  private AstmD6377Example() {}

  public static void main(String[] args) {
    SystemInterface sourceOil = new SystemSrkEos(275.15, 1.0);
    sourceOil.addComponent("methane", 0.0006538);
    sourceOil.addComponent("ethane", 0.006538);
    sourceOil.addComponent("propane", 0.065380);
    sourceOil.addComponent("n-pentane", 0.154500);
    sourceOil.addComponent("nC10", 0.545000);
    sourceOil.setMixingRule(2);
    sourceOil.init(0);

    SystemInterface workingFluid = sourceOil.clone();
    Standard_ASTM_D6377 vaporPressure = new Standard_ASTM_D6377(workingFluid);
    vaporPressure.setReferenceTemperature(37.8, "C");
    vaporPressure.setMethodRVP(RvpMethod.RVP_ASTM_D6377);
    vaporPressure.calculate();

    RvpResult selected = vaporPressure.getRvpResult();
    RvpResult vpcr4 = vaporPressure.getRvpResult(RvpMethod.VPCR4);
    RvpResult dryVpcr4 = vaporPressure.getRvpResult(RvpMethod.VPCR4_NO_WATER);
    double tvpBara = vaporPressure.getValue("TVP", "bara");
    double selectedRvpKPa = vaporPressure.getValue("RVP", "kPa");

    requireFinitePositive("selected RVP", selected.getValue());
    requireFinitePositive("VPCR4", vpcr4.getValue());
    requireFinitePositive("water-free VPCR4", dryVpcr4.getValue());
    requireFinitePositive("TVP", tvpBara);
    requireFinitePositive("selected RVP", selectedRvpKPa);

    logger.info("Selected result {}", selected.toJson());
    logger.info("TVP {} bara; VPCR4 {} bara; dry VPCR4 {} bara",
        tvpBara, vpcr4.getValue(), dryVpcr4.getValue());
    logger.info("Source state remains {} K and {} bara",
        sourceOil.getTemperature(), sourceOil.getPressure());
  }

  private static void requireFinitePositive(String name, double value) {
    if (!Double.isFinite(value) || value <= 0.0) {
      throw new IllegalStateException(name + " calculation failed: " + value);
    }
  }
}

When adding water or another component to an already configured fluid, reapply setMixingRule(...) after the addition so the binary interaction matrices include the new component, then call init(0).

For a fluid without water, VPCR4 and VPCR4_NO_WATER should agree within numerical tolerance. For a water-bearing fluid, the difference is a model sensitivity; it is not permission to discard measured water or emulsion effects.

Water-Contact Storage: A Declared Engineering Basis

Oil stored in a water-contact cavern can be assessed on an assumed water-saturated storage basis. This is a stated storage condition, not a universal property of oil or of every sample. The entrained droplet amount captured in a laboratory sample can be incidental to sampling. A reproducible engineering comparison can therefore separate the characterized dry hydrocarbon EOS from an independently validated pure-water saturation pressure, rather than let arbitrary input droplet water determine the reporting basis.

An E300 model tuned to hydrocarbon properties is not automatically validated for water vapor pressure. Validate the particular model; do not infer that all water-bearing EOS calculations are inaccurate. Computational water removal is not advice to dry a laboratory sample and does not restore light ends already lost during sampling or handling.

Preserve the Characterized Hydrocarbon Model

The dry basis must retain the relative hydrocarbon molar amounts, renormalized after water is excluded. Preserve the EOS class, alpha model and parameters, heavy-component properties, mixing rule, named-component BIPs and volume shifts in every allocated phase, not just phase 0. Preserve non-water nonhydrocarbons as well unless their exclusion is separately justified.

Do not replace a characterized E300 fluid with an unrelated PR/SRK reconstruction. In particular, addTBPfraction(...) followed by critical-property overrides only through fluid.getComponent(...) does not update every phase and can invalidate the density/volume-shift characterization. Do not reapply a database mixing rule to a tuned fluid simply to repair component indexing: that can discard fitted BIPs.

Component-removal limitation: the current removeComponent("water") path compacts the component list without remapping the EOS BIP matrix. If water is not the last component, a hydrocarbon pair can inherit the wrong interaction. The lazy VPCR4_NO_WATER route uses this removal path too; selecting that label is not a preservation guarantee. A synthetic regression with water between pentane and methane detects a BIP change from 0.0123 to 0.5 in each allocated phase. Reject such a dry basis. Supply a separately audited water-free version of the same characterization instead; this guide intentionally does not offer a universal removal helper. Removing last-listed water is covered by a per-phase preservation test for a synthetic SRK/TBP fluid, not a guarantee for every E300 file or software version.

Partial-Pressure Addition and Correlation Order

For the declared D6377-labelled engineering convention at 37.8 °C:

\[\mathrm{RVPE}_{\mathrm{water\,sat}} =0.834\left[\mathrm{VPCR4}_{\mathrm{dry}}+p^{\mathrm{sat}}_{\mathrm{water}}(37.8\,^{\circ}\mathrm{C})\right] =\mathrm{RVPE}_{\mathrm{dry}}+0.834\,p^{\mathrm{sat}}_{\mathrm{water}}(37.8\,^{\circ}\mathrm{C}).\]

The second equality requires $\mathrm{RVPE}{\mathrm{dry}}=0.834\,\mathrm{VPCR4}{\mathrm{dry}}$. Do not add the full water saturation pressure after conversion to RVPE. Dry-basis selection and output correlation are separate choices: VPCR4_NO_WATER returns raw dry VPCR4, not dry RVPE. The historical D323-82 offset is 0.0607 bar (6.07 kPa); all terms in any correlation must use consistent pressure units.

For an EOS bubble-point TVP at the specified temperature $T$, the analogous approximation is:

\[\mathrm{TVP}_{\mathrm{water\,sat}}(T) =\mathrm{TVP}_{\mathrm{dry}}(T)+p^{\mathrm{sat}}_{\mathrm{water}}(T).\]
Reference temperature Illustrative rounded pure-water pressure Added contribution
37.8 °C About 6.54 kPa = 0.0654 bar RVPE: about 5.45 kPa = 0.0545 bar
30 °C About 4.24 kPa = 0.0424 bar TVP at 30 °C: about 4.24 kPa = 0.0424 bar

These rounded numbers illustrate the arithmetic, not a new NeqSim water-property API. Obtain $p^{\mathrm{sat}}$ from an approved independent source, for example an implementation of IAPWS-IF97 Region 4 within its saturation-temperature range (273.15–647.096 K), or an appropriate NIST water property source. Record the source, version, validity range, temperature and units. The 0.0424 bar value is a 30 °C study convention, not a temperature-independent constant.

Existing API Example on an Audited Dry Fluid

auditedDryOil below is a previously verified water-free version of the source characterization. The earlier synthetic sourceOil is water-free and is also suitable for illustrating these calls. The example performs no parameter mutation and applies the storage convention outside NeqSim.

SystemInterface workingOil = auditedDryOil.clone();
Standard_ASTM_D6377 standard = new Standard_ASTM_D6377(workingOil);
standard.setReferenceTemperature(37.8, "C");
standard.setMethodRVP(RvpMethod.VPCR4_NO_WATER);
standard.calculate();
RvpResult dry = standard.getRvpResult();
if (!dry.isValid()) {
  throw new IllegalStateException("No finite positive dry VPCR4 result");
}
// Rounded external inputs, in bar; replace with the approved property source.
double waterPsat378Bara = 0.0654;
double rvpeWaterSatBara = 0.834 * (dry.getValue() + waterPsat378Bara);

neqsim.process.equipment.stream.StreamInterface dryStream =
    new neqsim.process.equipment.stream.Stream("Dry oil", auditedDryOil.clone());
double tvp30DryBara = dryStream.getTVP(30.0, "C", "bara");
double tvp30WaterSatBara = tvp30DryBara + 0.0424;

Stream.getRVP(T, temperatureUnit, pressureUnit) defaults to raw VPCR4, not 0.834 * VPCR4. Select "RVP_ASTM_D6377" explicitly in its four-argument overload for that correlation on the stream’s supplied composition. getTVP(...) is a bubble-point calculation on the supplied fluid; it does not remove water or add an independent saturation correction. TVP at 30 °C and RVPE at 37.8 °C are different quantities at different temperatures; there is no general ordering rule between them.

Numerical Checks and Reporting Limits

A finite positive RvpResult is not a convergence certificate. Check the physical volume ratio on the calculation fluid and require coexisting gas and liquid phases. Native TVfractionFlash uses density-initialized, volume-corrected fractions by default. A TP-flash bracketing/bisection cross-check must use the same corrected volume objective, not raw phase.getVolume() ratios. Use initPhysicalProperties("density") before getCorrectedVolumeFraction(...); equivalently, phase mass divided by initialized phase density gives the physical phase volume. For a two-phase oil/gas result the target is a gas volume fraction of 0.8, or gas/liquid volume ratio 4.0. Extra liquid phases require an explicit total-liquid-volume basis. A binary pentane/TBP screening case can return a positive pressure while missing the 0.8 target; reject it rather than relaxing the volume tolerance. A high-water synthetic oil can also return oil/aqueous phases without gas while isValid() remains true. The regression tests distinguish that rejected case from three trace-water cases that meet the gas/liquid corrected-volume target. These are numerical checks, not validation of aqueous phase behavior. Repeat these checks for each model and operating case.

The additive convention assumes ideal partial-pressure addition, pure-water activity near one and sufficient water/contact to establish saturation in storage. Salts and glycols change water activity. Finite vapor/liquid ratio, water dissolution, nonideal coupling and emulsion effects cannot be inferred from this addition. It is neither an ASTM apparatus simulation nor proof of laboratory equivalence, legal compliance or universally superior accuracy. Compare with relevant measurements before adopting it for a decision.

Report dry and water-saturated estimates separately, including the quantity (raw VPCR4, correlated RVPE or bubble-point TVP), reference temperature, absolute pressure unit, water basis, complete EOS provenance, water-property source, convergence evidence and assumptions. Compare a product limit only on the same quantity, temperature and water basis. isOnSpec() does not perform this assessment.

The synthetic API and preservation checks are in OilVapourPressureWaterBasisTest; the original workflow is exercised by AstmD6377DocumentationTest. These are software regressions, not fabricated laboratory validation data.

API Contract

Operation Supported use
setReferenceTemperature(value, unit) Converts the supplied temperature to the internal Celsius reference
setMethodRVP(RvpMethod) Selects the result returned as RVP
calculate() Populates TVP, VPCR4, and the two direct correlations
getRvpResult() Returns the selected structured result
getRvpResult(method) Returns a named structured result; water-free variants are evaluated lazily
getValue("RVP", pressureUnit) Converts the selected method result from bara
getValue("TVP", pressureUnit) Converts the EOS bubble-point result from bara
getMethodRVP() Returns the selected legacy method label

The legacy string setter remains available, but the enum rejects unknown methods before a calculation is interpreted. A structured result is valid only when its value is finite and positive. It does not establish laboratory repeatability, regulatory acceptance, or fitness for a specific product specification.

Engineering Validation Checklist

Before using a calculated vapor pressure:

  1. Preserve a characterized source fluid and run the standard on a clone.
  2. Confirm light ends were not lost during sampling or fluid preparation.
  3. Document heavy-end characterization, equation of state, mixing rule, and water treatment.
  4. Record the exact method label, reference temperature, and absolute pressure unit.
  5. Compare against representative laboratory vapor-pressure data over the operating envelope.
  6. Apply the actual product or custody-transfer limit outside isOnSpec().
  7. Treat discrepancies as model or characterization evidence, not as a reason to tune silently.