Steam Trap-Steam Distribution Piping System

A steam trap is an essential element of a steam distribution piping system. As steam flows through a pipe, it cools off due to heat losses and converts into hot water; this hot water is called condensate. The function of a steam trap is to bleed off hot water so that only steam remains in the system. In addition to condensate removal, a steam trap also performs air venting function, thereby increasing the thermal efficiency and reliability of the steam distribution system.

A typical steam trap assembly consists of a number of components which enable the successful operation of the system.

1.       Stop Valves: To control (stop or allow) the steam flow through the system.
2.       Strainer: To remove debris from the steam; if debris is not removed, it might damage the trap.
3.       Check Valve: It stops the condensate from flowing back to the system.
4.       Trap: To catch water and allow the steam to flow through.

In normal practice, strainer and check valves are built in with the steam trap. 

·         Under normal operation, bypass stop valve remains closed and the other two stop valves remain open. Steam flows through the trap, where condensate is removed and drained while the steam returns to the steam mains. In some plants, condensate is recovered and returned to the boiler feed water tank. This results in significant savings in terms of energy and water usage.

Measuring Hydrocarbon Dew Point of Natural Gas Fuel for Gas Turbine

Natural gas is used by a number of industries, most notably in power generation by gas turbines.

Gas turbine manufacturers will always provide specifications defining the quality of the natural gas fuel provided to the turbine - designed to prevent damage to the turbine and the additional problems that can result. These specifications can include a number of parameters, including pressure, flow, acceptable contaminant limits and gas temperature – frequently with the inclusion of the term ‘superheat’.

When operating modern DLN (Dry low NOx) turbines, the following of these guidelines is critical to avoiding severe damage to the turbine, and criteria such as superheat have been set up to help prevent this. Superheat is defined as the temperature 50°F (28°C) above the Hydrocarbon and water Dew Points of the fuel gas, so if the dew point of the gas is -12°C, then the gas should be heated to +10°C.

Prior to combustion, the gas is running at pipeline pressure, much too great for the gas turbine, therefore the gas must be expanded in order to be suitable for use. As the pressure of the gas drops, so does the temperature. If this Joule- Thompson effect drops the temperature below the HCDP, then liquids will condense inside the burner tubes of the turbine and the cans and nozzles coke up and become significantly less efficient, causing dramatically increased NOx emissions. If this situation is allowed to continue for a short time, the burner section will need to be rebuilt. This means a 3-5 day unplanned shut-down, a large crew on-site around the clock for the expensive rebuild and lost revenue and plant availability. This will dramatically impact the profitability of the plant.

Another seriously costly effect of condensation is flashback. This can be caused by hydrocarbon condensates, and the effect is for a flame to be held downstream of the burners, in the recirculation area. This region is not designed to withstand heat of this nature, and the metal temperatures will increase dramatically, frequently causing physical damage to the hardware.

Superheat is designed to help prevent either of these occurrences by ensuring that the gas never comes close to its HCDP. Natural gas fuel conditioning systems are most commonly used to heat the incoming gas, but this heat requires energy to generate, and if overheating due to a lack of awareness of the gas dew point occurs, then large costs can be incurred.

Measurement Techniques:

There are a number of different accepted methods for measuring HCDP,

1) The original technique being to use a cooled mirror dewscope. This requires a skilled operator to view a mirror over which the sample is flowed. The mirror is then cooled, and the temperature at which the first drops of condensation are viewed is noted.

2) Another method of determining the HCDP is by means of a gas chromatograph (GC). This method determines the concentrations of each hydrocarbon element (up to C12 in most cases), and, through an equation of state calculation, the condensing points of the quantities of each component present are identified and calculated to give a hydrocarbon dew point for the complete mixture. However, due to the limitations of the device, when analysing heavy hydrocarbon molecules the calculations of the HCDP can frequently be quite inaccurate, suggesting that the HCDP is drier than the actual value.

3) The alternative is to use an automatic, optical condensation dew-point analyser, such as the Michell Instruments Condumax II, Ametek. These devices functions in a similar manner to the Cooled mirror dewscope. The cell has an etched optical surface with a central conical depression which normally refracts light unevenly. An LED shines at this surface and a photo-detector looks at an image of the light shining back, which in dry conditions, appears as a ring of light. The photo-detector is focused on the light scattered into the centre of the ring. A thermoelectric peltier device cools the surface until condensates begin to appear. The condensates alter the reflective properties of the surface, with the circle of light around the perimeter intensifying, and the scattered light in the centre dispersing according to the amount of condensate on the mirror. The exact signal level can be accurately monitored by looking at the signal from the photodetector. The mirror temperature is recorded when the desired level of condensates are deposited. The setting of the device gives readings which are comparable to readings obtained by an experienced dewscope operator.

Centrifugal Pumps: Understanding Cavitation

Centrifugal Pumps: Understanding Cavitation

Operating a pump under the condition of cavitation for even a short period of time can have damaging consequences for both the equipment and the process.

 Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.

The condition of cavitation is essentially an indication of an abnormality in the pump suction system.

Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation means different things to different people. Some say when a pump makes a rattling or knocking sound along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure, etc.

In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.

Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.

Vapor bubbles are formed due to the vaporization of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles - Vaporous Cavitation.

Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system)-Gaseous Cavitation.

Both types of bubbles are formed at a point inside the pump where the local static pressure is less than the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous cavitation).

Vaporous cavitation is the most common form of cavitation found in process plants. Generally it occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of pump parts.

Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is increased to 6%, the pump starts cavitating.

Mechanism of Cavitation :

The phenomenon of cavitation is a stepwise process as shown in Figure 

The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?

Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.

Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.

Valve Sizing and Selection

Valve Sizing and Selection

Sizing flow valves is a science with many rules of thumb that few people agree on. In this article I'll try to define a more standard procedure for sizing a valve as well as helping to select the appropriate type of valve. **Please note that the correlation within this article is for turbulent flow.

Step #1: Define the System

The system is pumping water from one tank to another through a piping system with a total pressure drop of 150 psi. The fluid is water at 70 °F. Design (maximum) flowrate of 150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm. The pipe diameter is 3 inches. At 70 °F, water has a specific gravity of 1.0.

Key Variables: Total pressure drop, design flow, operating flow, minimum flow, pipe diameter, specific gravity

Step #2: Define a maximum allowable pressure drop for the valve

When defining the allowable pressure drop across the valve, you should first investigate the pump.  What is its maximum available head? Remember that the system pressure drop is limited by the pump. Essentially the Net Positive Suction Head Available (NPSHA) minus the Net Positive Suction Head Required (NPSHR) is the maximum available pressure drop for the valve to use and this must not be exceeded or another pump will be needed. It's important to remember the trade off, larger pressure drops increase the pumping cost (operating) and smaller pressure drops increase the valve cost because a larger valve is required (capital cost). The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater. For our system, 10% of the total pressure drop is 15 psi which is what we'll use as our allowable pressure drop when the valve is wide open (the pump is our system is easily capable of the additional pressure drop).

Step #3: Calculate the valve characteristic

For our system:

At this point, some people would be tempted to go to the valve charts or characteristic curves and select a valve. Don't make this mistake, instead, proceed to Step #4!

Step #4: Preliminary valve selection

Don't make the mistake of trying to match a valve with your calculated Cv value. The Cv value should be used as a guide in the valve selection, not a hard and fast rule. Some other considerations are:

a. Never use a valve that is less than half the pipe size
b. Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10-80% stroke range.

Before a valve can be selected, you have to decide what type of valve will be used (See the list of valve types later in this article). For our case, we'll assume we're using an equal percentage, globe valve (equal percentage will be explained later). The valve chart for this type of valve is shown below. This is a typical chart that will be supplied by the manufacturer (as a matter of fact, it was)

For our case, it appears the 2 inch valve will work well for our Cv value at about 80-85% of the stroke range. Notice that we're not trying to squeeze our Cv into the 1 1/2 valve which would need to be at 100% stroke to handle our maximum flow. If this valve were used, two consequences would be experienced: the pressure drop would be a little higher than 15 psi at our design (max) flow and the valve would be difficult to control at maximum flow. Also, there would be no room for error with this valve, but the valve we've chosen will allow for flow surges beyond the 150 gpm range with severe headaches!

So we've selected a valve...but are we ready to order? Not yet, there are still some characteristics to consider.

Step #5: Check the Cv and stroke percentage at the minimum flow

If the stroke percentage falls below 10% at our minimum flow, a smaller valve may have to be used in some cases. Judgments plays role in many cases. For example, is your system more likely to operate closer to the maximum flow rates more often than the minimum flow rates  Or is it more likely to operate near the minimum flow rate for extended periods of time. It's difficult to find the perfect valve, but you should find one that operates well most of the time. Let's check the valve we've selected for our system:

Referring back to our valve chart, we see that a Cv of 6.5 would correspond to a stroke percentage of around 35-40% which is certainly acceptable. Notice that we used the maximum pressure drop of 15 psi once again in our calculation. Although the pressure drop across the valve will be lower at smaller flow rates  using the maximum value gives us a "worst case" scenario. If our Cv at the minimum flow would have been around 1.5, there would not really be a problem because the valve has a Cv of 1.66 at 10% stroke and since we use the maximum pressure drop, our estimate is conservative. Essentially, at lower pressure drops, Cv would only increase which in this case would be advantageous.

Step #6: Check the gain across applicable flow rates

Gain is defined as:

Now, at our three flowrates:
Q_min = 25 gpm
Q_op = 110 gpm
Q_des = 150 gpm

we have corresponding Cv values of 6.5, 28, and 39. The corresponding stroke percentages are 35%, 73%, and 85% respectively. Now we construct the following table:

Flow (gpm)	Stroke (%)	Change in flow (gpm)	Change in Stroke (%)
25	35	110-25 = 85	73-35 = 38
110	73
150	85	150-110 = 40	85-73 = 12

Gain #1 = 85/38 = 2.2
Gain #2 = 40/12 = 3.3

The difference between these values should be less than 50% of the higher value. 0.5 (3.3) = 1.65 and 3.3 - 2.2 = 1.10. Since 1.10 is less than 1.65, there should be no problem in controlling the valve. Also note that the gain should never be less than 0.50. So for our case, I believe our selected valve will do nicely!

Other Notes

Another valve characteristic that can be examined is called the choked flow. The relation uses the FL value found on the valve chart. I recommend checking the choked flow for vastly different maximum and minimum flowrates. For example if the difference between the maximum and minimum flows is above 90% of the maximum flow, you may want to check the choked flow. Usually, the rule of thumb for determining the maximum pressure drop across the valve also helps to avoid choking flow.

Selecting a Valve Type

When speaking of valves, it's easy to get lost in the terminology. Valve types are used to describe the mechanical characteristics and geometry (Ex/ gate, ball, globe valves). We'll use valve control to refer to how the valve travel or stroke (openness) relates to the flow:

1. Equal Percentage: equal increments of valve travel produce an equal percentage in flow change
2. Linear: valve travel is directly proportional to the valve stoke
3. Quick opening: large increase in flow with a small change in valve stroke

So how do you decide which valve control to use? Here are some rules of thumb for each one:

1. Equal Percentage (most commonly used valve control)

a. Used in processes where large changes in pressure drop are expected

b. Used in processes where a small percentage of the total pressure drop is permitted by the valve

c. Used in temperature and pressure control loops

2. Linear

a. Used in liquid level or flow loops

b. Used in systems where the pressure drop across the valve is expected to remain fairly constant (ie. steady state systems)

3. Quick Opening

a. Used for frequent on-off service

b. Used for processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)

Now that we've covered the various types of valve control, we'll take a look at the most common valve types.

Gate Valves:

Best Suited Control: Quick Opening

Recommended Uses:

1. Fully open/closed, non-throttling 2. Infrequent operation 3. Minimal fluid trapping in line

Applications: Oil, gas, air, slurries, heavy liquids, steam, noncondensing gases, and corrosive liquids

Advantages:

1. High capacity , 2. Tight shutoff  3. Low cost  4. Little resistance to flow

Disadvantages:

1. Poor control, 2. Cavitate at low pressure drops, 3. Cannot be used for throttling

Globe Valves

Best Suited Control: Linear and Equal percentage

Recommended Uses:

1. Throttling service/flow regulation 2. Frequent operation

Applications: Liquids, vapors, gases, corrosive substances, slurries

Advantages:

1. Efficient throttling  2. Accurate flow control  3. Available in multiple ports

Disadvantages:

1.High pressure drop 2. More expensive than other valves

Ball Valves:

Best Suited Control: Quick opening, linear

Recommended Uses:

1. Fully open/closed, limited-throttling 2. Higher temperature fluids

Applications: Most liquids, high temperatures, slurries

Advantages:

1. Low cost  2. High capacity  3. Low leakage and maint. 4. Tight sealing with low torque

Disadvantages:

1. Poor throttling characteristics 2. Prone to cavitation

Butterfly Valves:

Best Suited Control: Linear, Equal percentage

Recommended Uses:

1. Fully open/closed or throttling services 2. Frequent operation 3. Minimal fluid trapping in line

Applications: Liquids, gases, slurries, liquids with suspended solids

Advantages:

1. Low cost and maint.  2. High capacity  3. Good flow control 4. Low pressure drop

Disadvantages:

1. High torque required for control 2. Prone to cavitation at lower flows

Other Valves

Another type of valve commonly used in conjunction with other valves is called a check valve. Check valves are designed to restrict the flow to one direction. If the flow reverses direction, the check valve closes. Relief valves are used to regulate the operating pressure of incompressible flow. Safety valves are used to release excess pressure in gases or compressible fluids.

 References

Rosaler, Robert C., Standard Handbook of Plant Engineering, McGraw-Hill, New York, 1995, pages 10-110 through 10-122

Purcell, Michael K., "Easily Select and Size Control Valves", Chemical Engineering Progress, March 1999, pages 45-50

Electrical Tomography Measurement Technique- Multi phase Flow

Electrical Process Tomography

What is Electrical Process Tomography?

Electrical Tomography is a measurement technique for obtaining information about the contents of process vessels and pipelines. Multiple electrodes are arranged around the boundary of the vessel at fixed locations in such a way that they do not affect the flow or movement of materials.  Tomographic measurement techniques differ from point measurement techniques, because they sample a substantial proportion of the process volume rather than at a single point. Circular pipeline-based sensors measure an entire cross-sectional volume.

The technology can be used for liquid/liquid, solid/ liquid, gas/liquid, gas/solid/liquid systems. The spatial resolution of the imaging method and the sensitivity of the method depend specifically on the electrical properties of the system being measured and upon the dimensions of the process.

Typically, a sensor consists of 16 electrodes and for research applications up to 8 x 16 electrodes may be arranged within a process vessel.

The technique can be used with a wide range of research and development applications demonstrated, including:

ü          interrogation of mixing processes

ü          investigating a solid-liquid filtration process

ü                  monitoring the performance of a hydro cyclone

ü                  measurement and control of bubble columns

ü                  measurement of multiphase flow

One of the main application areas of electrical tomography is the measurement of multiphase flow regimes.

Flow processes may involve a variety of phases or components in the gas, liquid or solid phase and are complex in their nature. Electrical tomography techniques provide the capability for flow visualization, regardless of material opacity, to enhance the understanding of such complex flow processes.

Measuring Solid-Liquid Flow Using Electrical Resistance Tomography

Electrical Resistance Tomography (ERT) met the criteria due to the robust and simple nature of the equipment and the absence of any radioactive, cryogens or dangerous components means the equipment can be readily employed at mine sites both above and below ground. Measurements were performed on a 100 mm diameter flow loop with closely graded 2 mm silica sand suspended in clear shear thinning polymer suspensions. These ‘model' suspensions mimic the behaviour of bimodal suspensions of particles containing a large fraction of fine rheologically active particles that would form a non-Newtonian carrier in which would be suspended the coarser fractions such as those found on mining co-disposal lines.

Figure 1 shows a comparison between a photograph of the actual pipe flow and ERT derived concentration maps using on-line single step Linear Back Projection (LBP) algorithm and off-line iterative Sensitivity Conjugate Gradients (SCG) algorithm. The LBP algorithm has the advantage that images are produced on-line at rates of multiple images per second. However, it can be seen that there is some blurring of the solid-liquid interface. The SCG algorithm is in substantial agreement with actual pipe flow conditions.

Fig 1

Rules of Thumb for Process Engineers

Experience is typically what turns a good engineer into a great engineer. An engineer that can look at a pipe and a flowmeter and guess the pressure drop within 5%. Someone who can at least estimate the size of a vessel without doing any calculations.

When I think of such rules, two authors come to my mind, Walas and Branan. Dr. Walas' book, Chemical Process Equipment: Selection and Design has been widely used in the process industry and in chemical engineering education for years. Mr. Branan has either helped write or edit numerous books concerning this topic. Perhaps his most popular is Rules of Thumb for Chemical Engineers. Here, I'll share some of these rules with you along with some of my own. Now, be aware that these rules are for estimation and are not necessary meant to replace rigorous calculations when such calculations should be performed. But at many stages of analysis and design, these rules can save you hours and hours.

Physical Properties

Property	Units	Water	Organic Liquids	Steam	Air	Organic Vapors
Heat Capacity	KJ/kg 0C	4.2	1.0-2.5	2.0	1.0	2.0-4.0
	Btu/lb 0F	1.0	0.239-0.598	0.479	0.239	0.479-0.958
Density	kg/m3	1000	700-1500		1.29@STP
	lb/ft3	62.29	43.6-94.4		0.08@STP
Latent Heat	KJ/kg	1200-2100	200-1000
	Btu/lb	516-903	86-430
Thermal Cond.	W/m 0C	0.55-0.70	0.10-0.20	0.025-0.070	0.025-0.05	0.02-0.06
	Btu/h ft 0F	0.32-0.40	0.057-0.116	0.0144-0.040	0.014-0.029	0.116-0.35
Viscosity	cP	1.8 @ 0 0C	**See Below	0.01-0.03	0.02-0.05	0.01-0.03
		0.57 @ 50 0C
		0.28 @ 100 0C
		0.14 @ 200 0C
Prandtl Number		1-15	10-1000	1.0	0.7	0.7-0.8

Material	Advantage		Disadvantage
Carbon Steel	Low cost, easy to fabricate, abundant, most common material. Resists most alkaline environments well.		Very poor resistance to acids and stronger alkaline streams. More brittle than other materials, especially at low temperatures.
Stainless Steel	Relatively low cost, still easy to fabricate. Resist a wider variety of environments than carbon steel. Available is many different types.		No resistance to chlorides, and resistance decreases significantly at higher temperatures.
254 SMO (Avesta)	Moderate cost, still easy to fabricate. Resistance is better over a wider range of concentrations and temperatures compared to stainless steel.		Little resistance to chlorides, and resistance at higher temperatures could be improved.
Titanium	Very good resistance to chlorides (widely used in seawater applications). Strength allows it to be fabricated at smaller thicknesses.		While the material is moderately expensive, fabrication is difficult. Much of cost will be in welding labor.
Pd stabilized Titanium	Superior resistance to chlorides, even at higher temperatures. Is often used on sea water application where Titanium's resistance may not be acceptable.		Very expensive material and fabrication is again difficult and expensive.
Nickel	Very good resistance to high temperature caustic streams.		Moderate to high expense. Difficult to weld.
Hastelloy Alloy	Very wide range to choose from. Some have been specifically developed for acid services where other materials have failed.		Fairly expensive alloys. Their use must be justified. Most are easy to weld.
Graphite	One of the few materials capable of withstanding weak HCl streams.		Brittle, very expensive, and very difficult to fabricate. Some stream components have been know to diffusion through some types of graphites.
Tantalum	Superior resistance to very harsh services where no other material is acceptable.		Extremely expensive, must be absolutely necessary.

Power = m z1 R T1 [({P2 / P2}a - 1)] / a

Eq. (4)

T₁ is the inlet temperature
R is the gas constant
z₁ is the compressibility
m is the molar flow rate
a = (k-1)/k
k = Cp/Cv
65% at compression ratios of 1.5 
75% at compression ratios of 2.0 
80-85% at compression ratios between 3 and 6
k =0.20 for P >90 torr, 0.08 for 3 < P < 20 torr, and 0.025 for P < 1 torr
V = equipment volume in cubic feet
Leakage = air leakage into equipment in lb/h

** Viscosities of organic liquids vary widely with temperature

Liquid densities vary with temperature to this approximation

Materials of Construction

Compressors and Vacuum Equipment

A. The following chart is used to determine what type of compressor is to be used:

B. Fans should be used to raise pressure about 3% (12 in water), blowers to raise to less than 2.75 barg (40 psig), and compressors to higher pressures.

C. The theoretical reversible adiabatic power is estimated by:

where: 

D. The outlet for the adiabatic reversible flow, T₂ = T₁ (P₂ / P₁)a

E. Exit temperatures should not exceed 204 °C (400 °F).

F. For diatomic gases (Cp/Cv = 1.4) this corresponds to a compression ratio of about 4

G. Compression ratios should be about the same in each stage for a multistage unit, the ratio = (Pn / P1) 1/n, with n stages.

H. Efficiencies for reciprocating compressors are as follows: 

I. Efficiencies of large centrifugal compressors handling 2.8 to 47 m3/s (6000-100,000 acfm) at suction is about 76-78%

J. Reciprocating piston vacuum pumps are generally capable of vacuum to 1 torr absolute, rotary piston types can achieve vacuums of 0.001 torr.

K. Single stage jet ejectors are capable of vacuums to 100 torr absolute, two stage to 10 torr, three stage to 1 torr, and five stage to 0.05 torr.

L. A three stage ejector requires about 100 lb steam/lb air to maintain a pressure of 1 torr.

M. Air leakage into vacuum equipment can be approximated as follows: Leakage = k V(2/3)

where: