Test methods

Straight pipes

The lambda value depends on the temperature. It requires measurements to be taken at at least three different temperatures. The measurements are taken on a section of DN 50/125–140 pre-insulated pipe that is at least three metres long. This section is taken from a 12-metre section of pipe from the standard production line. The ambient temperature (ta) is kept constant while the measurements are performed.

The temperature on the outer casing is measured when the temperature of the carrier pipe is constant. The lambda values for the three different temperature conditions can be calculated on the basis of the temperatures of the carrier pipe, outer casing and surroundings.

The lambda value for a pre-insulated district heating pipe is always stated as an average temperature measured in the polyurethane foam at a temperature of 50°C. A straight line can be traced from the calculated lambda values, as the lambda value is directly proportional to the temperature. The lambda value at 50°C can then be read off the curve for temperature-related lambda values.

FlexPipe

The lambda value depends on the temperature. It requires measurements to be taken at at least three different temperatures. The measurements are performed on a roll of FlexPipe measuring (approx.) 25 metres. The precise dimensions depend of the type of carrier pipe in the roll of FlexPipe. The length of pipe is taken from a complete roll from the normal production line. The ambient temperature (ta) is kept constant while the measurements are performed.

On account of the length and size of the pipe section on which the measurement is taken, it is not possible to keep the temperature of the carrier pipe constant. Instead, water at a constant inflow temperature (ti) is circulated through the pipe. The temperature for the outer casing is measured when the inflow (ti) and outflow (tu) temperatures for the carrier pipe are constant. The measurements are performed at three different carrier pipe temperatures. The lambda values for the three temperatures can then be calculated on the basis of the inflow temperature (ti), the outflow temperature (tu), the temperature of the outer casing, and the ambient temperature. ​

The lambda value for a pre-insulated district heating pipe is always stated as an average temperature measured in the polyurethane foam at a temperature of 50°C. A straight line can be traced from the calculated lambda values, as the lambda value is directly proportional to the temperature. The lambda value at 50°C can then be read off the curve for temperature-related lambda values.