L

The law of congruence describes the congruity between two states or circumstances (see also Pump affinity laws).

The abbreviation LCC stands for "life cycle costs".

LCD stands for "Liquid Crystal Display".

It is a term that describes an electronic display for graphically depicting or visualising data and information. LCDs are used in communications systems, among other applications.

Leakage loss refers to the volume flow rate leaking from the shaft seals (Q_L).

LED stands for "Light Emitting Diode". It is a semiconductor diode that produces a clearly defined spectrum of light and is used in communications systems.

The term level is used in metrology to characterise the level of a physical measurand. It is used in water, sound, fill, signal, and voltage level applications.

Life cycle costs (also see LCC) are the total costs incurred throughout the service life of a pump system. They are used to compare the economic efficiency of various technical designs. The LCC equation is determined in accordance with the guidelines of EUROPUMP and the Hydraulic Institute.

The life cycle costs associated with the operation of a pump or pump system are determined by calculating the annual costs of operation plus the interest and depreciation for the non-current assets, like the machinery and buildings, for a variety of alternatives.

In the case of centrifugal pumps, energy (C_e), operating (C_o) and maintenance costs (C_m) account for the largest proportion of the life cycle costs.
See Fig. 1 Life cycle costs

The energy costs are calculated as follows:

The calculation applies to one specific operating point only. Given that pump operation usually involves a broad flow rate range, a pro rata calculation must be performed for the various flow rates involved. The individual results are then added together, taking the load profile into account.

The costs of operation and maintenance must be determined on a case-by-case basis and are dependent on the level of automation, operating period and maintenance requirement of the system.

LCC analysis − Key financial factors

• Energy price increase (inflation)
• Interest and discount rate
• Expected system life (calculation period)

Calculating the current costs associated with a specific cost element:

Based on empirical evidence, the following factors can be derived for dimensioning an economically efficient pump system or piping:
See Fig. 2 Life cycle costs

• Having high flow velocities in narrow piping reduces the cost of the system, but increases energy requirements and wear. 
• If operating periods are long, the energy costs are the dominant factor. This means that any extra costs for energy-saving measures such as speed adjustment (see Closed-loop control) pay off very quickly.
• If operating periods are short, investments should be low and flow velocities can be high.
• A smaller number of fairly large pumps often produces higher levels of efficiency, resulting in lower energy costs. However, as the number of redundant pumps increases so does the capital expenditure.

The lift check valve is a valve which prevents the suction line from running empty, e. g. after the pump has been stopped. Priming the pump prior to a re-start is therefore no longer necessary.

Spring-loaded lift check valves can be installed in the piping in almost any mounting position. Compared with swing check valves their  head losses, are higher as the deflection of the flow is much more significant. Lift check valves are preferred when applications require smaller nominal diameters.

A special lift check valve design is the automatic recirculation or minimum flow valve. See Fig. 1 Lift check valve

A lined pump is also referred to as a slurry pump.

The liquefied gas pump is a centrifugal pump used for handling liquefied hydrocarbons, such as propane, butane, propylene and ethylene, in refineries in the distillation of crude oil. These come under the general heading of LPG (Liquefied Petroleum Gas) and are pumped at about minus 104 ºC.

The liquefied gas pump is also used for handling liquefied natural gas consisting mainly of methane. This is generally known as LNG (Liquefied Natural Gas) and is handled at temperatures around minus 161 °C.

Depending on the application, various types of pumps are used in plants for the liquefaction, vaporisation, filling and storage of LPG and LNG, for example horizontal single or multistage refinery pumps of back pull-out design  see Fig. 1 Liquefied gas pump

or variants of these type series in the form of vertical pumps.
See Fig. 2 Liquefied gas pump, and Fig. 2 Refinery pump

Multistage, mainly vertical can-type pumps are used for higher heads The shaft passages on these are equipped with mechanical shaft seals.

Depending on the operating conditions, either single metal-bellows mechanical seals or double balanced mechanical seals arranged back to back and with barrier fluid are employed.

As liquefied gases can usually only be pumped at temperatures close to their boiling point (see Vapour pressure) the NPSH required is of great importance (see Suction characteristics). Thus for conventional centrifugal pumps, the NPSH is usually insufficient to enable the tanks to be emptied down to a level of only a few centimetres. This is why it often becomes necessary to use inducers to empty tanks. See Fig. 1 Inducer and inlet conditions

All wetted metal parts of liquefied gas pumps are made of suitable low-temperature materials. Apart from chrome nickel steels, aluminium alloys are also used.

The term liquid ring pump describes a group of pumps which are used as rotary positive displacement pumps for handling gases. The positive displacement element is an eccentric rotary liquid ring. Since it consists mostly of water, the water ring pump is a prime example.

The liquid-metal pump is a centrifugal pump, which because of its design is capable of handling liquid metals (at high temperatures) by means of heat barriers, special shaft seals and the use of suitable materials. A simple liquid metal pump is the mercury pump.

The live-zero signal is a standard signal that serves as a current or voltage signal for the analog transmission of a process variable to the PLC, for example.

Current signals to DIN IEC 60381-1

• 0 mA ... 20 mA 
• 4 mA ... 20 mA ("live-zero")

Voltage signals to DIN IEC 60381-2S

• 0 V ... 10 V 
• 2 V ... 10 V ("live-zero")

Live-zero signals are currently used in almost all industrial applications, whereby a non-zero electrical signal is assigned to the start of the measuring range, enabling monitoring for broken wires.

In this example, the non-zero starting signal of 4 mA or 2 V is the "live zero", whereas signals 0 mA and 0 V always point to a fault.
Since current signals are less sensitive to electromagnetic interference as occurs when adjacent consumers are activated, and voltage loss caused by line resistance is anticipated, this type of signal is preferred over voltage signals.

LNG stands for liquefied natural gas. It is cooled to −161 °C (boiling point at atmospheric pressure) and, in its liquid state, has a volume of only 1/600th of the natural gas volume in gas form. It consists mainly of methane and is almost always combined with CO₂ and N₂ (also see Liquefied gas pump).

The term load profile relates to structural durability and is the sum total of all loads, induced by torque, rotational speed, acceleration, rotational velocity and temperature that are acting on a component over a defined period.

Oscillating loads rarely occur on one level, i.e. with only one amplitude, but have many different amplitudes that occur in random sequence. Several load profiles are combined to create a collective load profile.

LON stands for"local operating network" and a LON bus refers to an open bus system that allows components from different manufacturers to be integrated in plants. LON buses are used in communications systems.

A loop is a closed circuit (also see Closed-loop test).

The loss coefficient (ζ) is a dimensionless number (characteristic coefficient) to calculate the head loss (H_L) (see Pressure loss): 

v     Characteristic flow velocity in the relevant hydraulic component (usually the flow velocity in the cross-section of the connection downstream of the component)

g      Acceleration due to gravity

In low flow operation the volume flow at the current operating point is lower than that at the design point (also see Operating behaviour).

Low flow protection devices are intended to protect the pump against impermissible low flow operation e.g. by installing a minimum flow controller (see Fig. 5 Valve) which helps to ensure a certain minimum flow. It is used in conjunction with boiler feed pumps.

Within the EU, the most important document for regulating the safety of electrical devices, apart from the EMC Directive, is the Low Voltage Directive (Directive 2006/95/EC). Save for a few exceptions, it covers "electrical equipment designed for use with a nominal voltage of between 50 and 1000 V for alternating current and between 75 and 1500 V for direct current".

The directive stipulates that the electrical equipment must be manufactured in such a way that it does not jeopardise the safety of people or domestic animals or place property at risk, assuming that it is installed and maintained properly and is used in accordance with its intended purpose

Low-pressure pumps are centrifugal pumps with heads up to 80 m (e.g. chemical pumps). Alternatives for other heads are the medium-pressure, high-pressure and super-pressure pumps.

LPG stands for liquefied petroleum gas and is a liquefied form of petroleum gas, or liquefied gas in general. It can be liquefied under light pressure and is a by-product of the refining process during distillation. It returns to its gaseous state when depressurised.

The boiling temperatures at atmospheric pressure are between -0.5 ºC (n-butane) and -61 ºC (n-butylene).

This gas is known in the mineral oil industry as LPG, which (under normal conditions) means the gaseous hydrocarbons propane and butane, and their gaseous olefins such as propylene and butylene, and combinations thereof.

Propane and butane are growing in importance as combustible gases in domestic, commercial and industrial applications, whilst the corresponding gaseous olefins are used preferably in the petrochemical industry.

Transport (see Liquefied gas pump) is accomplished in pressurised bottles for small quantities, and in pressure tank cars or trucks for larger quantities.