I have had experience of pipeline blending heavy Alberta Crude with distillates before it can be shipped via the pipeline. SInce the distillates were returned from the destination refineruy for the crude there was not a compatibility problem but asphaltene precipitation can be and this was noticed in some instalations. Since these blends were controlled using both density and viscosity, the viscometers initially suffered from coating but redesigning the viscometer installation and PFA coating them solved the problem. Noticably, the further downstream from the blend point, the less affected by precipitation.
Asphaltene precipitaion occurs as the resins that bind together the asphaltenes and maltenes are soluble in the light fractions.
As a general rule, the greater the difference in viscosity/density the more problems to be anticipated.
However, a concern when blending any crudes is compatability and stability.
Some crudes are not compatible with each other and the results are referred to as "Cocktails". They will separate out when you least want it.
Heavy fuel oils represent a similar situation and you may find good advise on the IBIA website, VISW labs site and others dealing with fuel oil blending.
One point to be sure of is to make sure the crudes are properly mixed. Too many problems occur through inadequately mixed blends that subsequently separate whether they are compatible or not.
As Reena 1957 states, inline blending is to be preferred. You might visit the
wbsite for details on crude blending. Their blenders use jet mixers to ensure a good blend. Some operators rely on pipeline blending but an example quoted is of a large bore pipeline into which well water and reservoir water were introduced (discriminated by their hardness values) and they were found to be flowing as separate streams in the same pipe some 500feet downstream.
These were almost identical fluids. Light and heavy crudes are not and they tend to laminar flows anyway. Using jetmixers or custody transfer static mixers is a must.
Inline blending is increasingly preferred because today we have the process instruments for density and viscosity which provide for very tight closed loop control. The refinery standard process capillary is less suitable due to its inability to handle the untreated product and slow response times.
Modern instruments make inline blending both practical, accurate and cost efficient.
I couldn't tell you how much the cost benefit was to the ALberta operation using a fiscal density meter and oonline digital viscometer for inline blend control but it was a huge saving in distilates.
JMW
