Steam

Written by Jerry Ratzlaff on . Posted in Thermodynamics

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Steam ( \(STM\) ) is the invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.  As water is heated and approaches its boiling point, some of the molecules attain kinetic energy enough to escape into the space above the surface of the liquid.  The more the water is heated the more molecules excapes.  When more molecules leave the liquid than enter the liquid, the saturation point is reached.  As the temperature continues to rising it reaches superheated steam where no liquid exists.

Steam Uses

Steam is used for many things in piping design.  In the oil and gas industry, steam is used to heat production tanks, keep flow lines from freezing and to increase oil production in formations that are very viscious.  In other industries, it can be used to sterilize & clean equipment, sterilize equipment, and in reboilers which maintain tight tolerances on their temperatures.  Because of this, it is important to ensure that the line has been sized properly to reduce pressure drop and to also ensure that right steam quality, pressure and temperature is being delivered where it is supposed to.  Sizing a line too small will cause the water vapor and liquid to travel too fast which will reduce overall pressure in the line.  If the line is too large, the fluid and vapor will travel slower and heat loss will occur.  Equally important to sizing a line, sizing a valve for use in steam service.

Steam Generation

This article looks at steam created by the combustion of natural gas or other hydrocarbons.  It may be created using other technologies such as solar or nuclear.   In a steam generator or boiler, water at a high pressure flows through piping which is exposed to a flame and heats the liquid water to a mixture of liquid water and gaseous water.  The amount of liquid in a steam line is called the steam quality.  Water that is 100% quality is 100% vapor or called 'dry steam.  Water that is 70% quality is 30% liquid and 70% vapor.  If heat is added beyond the point of 100% quality steam, superheated steam may form. 

In general, it is desireable to have higher quality steam than lower quality.  This is because water vapor holds more energy than its water counterpart at the same temperature.  

Steam Flow Regimes

Because steam can behave as a liquid and as a gas and everything inbetween, the way steam flows through a pipe can change through long straight runs.  Two phase flow regimes are very complicated and often require specialty programs to determine how the fluid is expected to flow.  For example, the liquid in very wet steam will move slower than its vapor counterpart.  High flowing velocites in the vapor space might cause liquid slugging that might damage equipment or piping.  If a wet steam line is flowing uphill, there could be two directional flow in the pipe asa the vapor travels uphill and the liquid travels downhill.  This may cause large wetspots in the line which use energy to overcome.   The ideal flow regime for steam is called annular flow where the liquid is moving along the entire ID of the pipe and the vapor travels down the middle.  During this flow, the flow it is very predictible and easy to operate. 

Steam Tables

Dry Steam

Dry steam ( \(DS\) ) does not contain water held in suspension.  It is similar to superheated steam.

Flash Steam

Flash steam ( \(FS\) ) is when hot condensate is released from a high pressure to a lower pressure steam.  This steam is the same as normal steam, the name is just given to explain how the steam is formed.

Flash Steam formula

\(FS =   \frac  {  h_i   \;   -   \;   h_o   }  { L }   \)

Where:

\(FS\) = flash steam %

\(h_i\) = specific enthalpy of saturated water at inlet

\(H_o\) = specific enthalpy of saturated water at outlet

\(L\) = latent heat of saturated water at outlet

High Pressure Steam

High pressure steam ( \(HPS\) ) is when the pressure greatly exceeds that of the atmosphere.

Low Pressure Steam

Low pressure steam ( \(LPS\) ) is when the pressure is less than or equal to that of the atmosphere.

Saturated Steam

Saturated steam is the point (temperature and pressure) when steam is in contact with the liquid water (boiling) it came from.  This steam contains small quanities of water and is considered to be wet steam.

Steam Density

Steam Density FORMULA

\(\rho_s = \frac{P\; \dot \;\; MW}{R^*T}\) 

Where:

\(\rho\) (Greek symbol rho) = steam density

\(P\) = pressure

\(MW\) = molecular weight of water

\(R^*\) = universal gas constant

\(T\) = temperature

STEAM QUALITY

Steam quality or steam dryness is the porportion of saturated steam (vapor) in a saturated condensate (liquid) / steam (vapor) mixture.  A steam quality of 0 indicates 100% liquid (condensate), a quality of 100 indicates 100% steam.  One (1) pound of steam with 95% steam and 5% of liquid has a steam quality of 0.95.

The measuremenrs needed to obtain a steam quality measurement are temperature, pressure, and the entrapment of liquid content.  There is a proportional relationship between temperature and pressure in saturated steam, as the temperature rises so does the pressure.

Superheated Steam

Superheated steam is steam at any given pressure which is heated to a temperature higher than the temperature of saturated steam. It can not contain water or have water exist at this pressure and resembles a gas.  In order to obtain this type of steam, steam is heated to a temperature of 100 °C or higher under normal pressure and has a higher heat transfer capability.  The properties of superheated steam are closer to a gas than a vapor.  With superheated steam temperature and pressure are independent variables not like saturated steams proportional relationship.

Wet Steam

Wet steam contains both water and steam held in suspension just below the satutation temperature

wet steam dryness fraction FORMULA

\(\zeta = \frac { w_s }  { w_w \; + \; w_s } \)

Where:

\(\zeta \) (Greek symbol zeta) = dryness fraction

\(w_s \) = mass of steam

\(w_w\) = mass of water

wet steam enthalpy FORMULA

\(h_t = h_s  \;  \zeta  \; + \;  \left(  1  \; - \;  \zeta  \right) h_w   \)

Where:

\(h_t \) = enthalpy of wet steam

\(h_s \) = enthalpy of dry steam

\(\zeta \) (Greek symbol zeta) = dryness fraction

\(h_w\) = enthalpy of saturated water of concensate

wet steam ENTropy FORMULA

\(s_t = s_s  \;  \zeta  \; + \;  \left(  1  \; - \;  \zeta  \right) s_w   \)

Where:

\(s_t \) = entropy of wet steam

\(s_s \) = entropy of dry steam

\(\zeta \) (Greek symbol zeta) = dryness fraction

\(s_w\) = entropy of saturated water of concensate

wet steam FLOW COEFFICIENT FORMULA

\(C_v = C_{vs} \; \dot \; \;  \zeta ^ { \frac { 1 }  { 2 } } \)

Where:

\(C_v \) = saturated wet steam flow coefficient

\(C_{vs} \) = saturated flow coefficient

\(\zeta \) (Greek symbol zeta) = dryness fraction

wet steam specific volume FORMULA

\(\upsilon_t = \upsilon_s  \;  \zeta  \)

Where:

\(\upsilon_t \) = specific volume of wet steam

\(\upsilon_s \) = enthalpy of dry steam

\(\zeta \) (Greek symbol zeta) = dryness fraction

 

Tags: Equations for Steam Equations for Pipe Sizing