Perspectives: Field Notes

Pumps: Check "Standard Conditions"

Otherwise you risk using the wrong value for density in calculations.

By Dirk Willard, Contributing Editor

"The pump discharge pressure will change depending on the liquid," said the client's maintenance engineer. For the sake of decorum, I remained quiet and kept a poker face despite knowing he was wrong. For years, I had specified pumps in psi not head to avoid such confrontations. This and other misconceptions seem to run amok in engineering.

First, let's consider the pump head. In my opinion, head, where used, should be in feet of water at 68°F. Those are the conditions the Hydraulic Institute, an association of pump makers and users, chooses when it talks about pumps. The trouble with many pump curves is that they imply water is at 68°F but never state that. This leaves room for mischief.

When you have discharge head (DH) and suction head (SH) in pressure, e.g., psi, the total dynamic head (TDH) is a fixed quantity (TDH = DH – SH). The pressure depends upon the specific gravity (SG) of the liquid versus that of water at 68°F. You might argue that using the density of water at 60°F would be more appropriate if the operating company uses that basis — but, remember, we're ordering a pump! If the head is in ft-liquid, as it is during energy balance calculations, then the pressure, unless converted, isn't known yet. If stated as a result in pressure, the value remains constant.

Water density introduces another potential snag. Water at 68°F has a specific gravity of 0.9983, not 1.0000, so let's discuss that misconception. "Cameron Hydraulic Data," 19th ed., takes considerable pains at the beginning of Section 4 to define the specific gravity of water. The oil industry, including the American Petroleum Institute, uses 60°F as the reference temperature for specific gravity; simulation results for "standard conditions" reference 60°F. Water's specific gravity at 60°F is 0.9991; its density is 62.367 lb/ft3, according to the U.S. National Institute of Standards and Technology (NIST). In the International System of Units, the reference temperature is 25°C (77°F); water has a specific gravity of 0.9971 and a density of 62.25 lb/ft3 at that temperature. Other groups use 4°C (39.2°F), the temperature at which the specific gravity actually is 1.000, the maximum value; density is 62.425 lb/ft3. As "Cameron Hydraulic Data" and other engineering books point out, the variety of so-called standard conditions can cause confusion. So, it's best to define the reference conditions at every use, regardless of how pedantic that may seem.

While we're on the topic of the misuse of specific gravity, let's discuss density calculations with vapors. Gas specific gravity often is defined in terms of the gas density divided by the density of air at a standard condition (SC): SG = ρgasair-SC. Numerous online references use cute shortcut methods, e.g., SG = molecular weight of the gas/molecular weight of air. This definition is at best sloppy engineering and at worst completely erroneous. If the temperature increases, the density should decrease but won't based on this unsound over-simplification, especially when the compressibility factor, Z, is not 1. The "Standard Handbook of Petroleum and Natural Gas Engineering," 2nd ed., p. 2-19, describes gas specific gravity as the density of the gas at a specified temperature and pressure divided by the density of air at a standard condition. Again, there's quite a variety of SCs.

The NIST and the International Union of Pure and Applied Chemistry take 1 bar (0.987 atm, 14.504 psia) at 0°C (32°F) as standard pressure and temperature (STP) while acknowledging informal standards for ambient temperature and pressure (SATP). NIST uses 20°C (68°F) at 1 atm for SATP. The gas industry relies on so-called International Standard Metric Conditions, 1 atm (14.696 psia) and 15°C (59°F), for natural gas and other refinery gases. For a listing of SCs of some organizations and government entities, including the U.S. Environmental Protection Agency, see: http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure. Another useful source is: www.thefreelibrary.com/A+twenty-first+century+molar+mass+for+dry+air.-a0186399836.

Don't confuse STP and SATP with standard state used in thermodynamics. Enthalpy, heat capacity and entropy all require a reference state. Most tabular data for these thermodynamic functions are at 25°C (77°F), although 0°C also has been used as reference. The gas pressure at thermodynamic reference state usually is such that the gas behaves as perfect gas.


dirk.jpgDIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at dwillard@putman.net

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